The original Halloween is noticeably different from its sequels and remake in a couple of ways:
There's much less blood and killing in the first film, which focuses far more on suspense. Starting with Halloween II Michael's kills were bloodier and more elaborate to more closely match the numerous slasher movies the original had inspired.
In the first movie, Michael has no apparent motivation; he's just "pure evil" and kills on instinct. In Halloween II, it's revealed that Laurie, his main target in the previous film and that one, is actually his sister. From then on out, every movie he appears in includes an element of him specifically stalking a family member. Taken to its illogical extreme in the sixth movie, which reveals Michael suffers from a druid curse that requires him to kill off his entire family.
For a monster that is now-infamous for its blinding-fast speed, the first Alien is a slow, almost shambling creature that can easily be outrun if you don't just stand there and let it slowly grab and bite you like that idiot Lambert did.
Star Wars Episode IV - A New Hope has a number of elements that can seem strange in comparison to the series as a whole.
The film has a number of plot elements that were changed or outright retconned in the later canon, with Anakin and Vader as different people, the Jedi and the Force being widely considered a myth, etc. A video showcasing all this exists here.
Darth Vader showed far more emotion when delivering his lines than he would in the sequels - he actually showed anger and rage when tearing through the Rebel ship and later showed amusement when Force choking an uppity Imperial during a meeting.
The lightsaber battle in Star Wars Episode IV - A New Hope is quite stiff and awkward compared to the more dramatic and gymnastic swordfights in later installments. According to Mark Hamill, the original lightsaber props had motors to rotate the "blades," making them too heavy and dangerous for any impressive flynning. Also, Obi-Wan's lightsaber "flickers" in mid-battle, while most subsequent lightsabers do not.
Jabba the Hutt wasn't a slug monster in the original screenplay, he was supposed to be a furry creature like Chewbacca, and the only scene in the original movie where he appears was cut. It was restored in the Special Edition, but Jabba appears much more mobile and less bulky in his CGI form than his puppet form from Return of the Jedi. This is because he was inserted into the footage over the stand-in actor, who was much shorter than Harrison Ford. Han calls Jabba a "wonderful human being," a sentiment that seems quite odd when said about a slug (although most viewers of the Special Edition assumed that Han was just trying to be sarcastic). Additionally, "Hutt" was not intended as a species name but a title, much like a mafia boss would be called Don.
Vader's suit was dirtier in the first movie and the metal part on his chest was behind the two cloth strips instead of in front of them. The eyepieces on his helmet were also tinted a very dark red before switching to solid black in Empire and beyond.
In the original cuts of Episode IV conventional English script could be seen, most notably when there's a closeup of a control panel showing the Death Star's tractor beam power dropping to zero (the words POWER and TRACTOR BEAM are clearly visible). In Episodes V and VI more care is taken to use alien-looking script, and the remastered versions of Episode IV substitute the visible English with this same alien lettering.
Obi-Wan refers to Vader as "Darth" during their fight rather than "Vader". Remember, this was before you-know-what was established, so at the time that actually was his name, not a title.
In the original Friday the 13th (1980), Jason not only isn't the killer, he doesn't even appear save for a dream sequence. He doesn't get his trademark hockey mask until Part III. Also, in Part 2, he's considerably less physically imposing than subsequent movies.
In the original films with Jason as a killer, he was alive rather than the indestructible undead human of later films. In the second and third films, he would run after his victims, back away if someone came at him with something dangerous, and would sometimes even grunt in pain. Surprisingly, despite not yet having his undead killer status yet in Friday the 13th: The Final Chapter, his behavior is actually closer to as it is in the later films.
Throughout the films, more and more areas are added to Hogwarts, making the Hogwarts of the first film almost a kind of bare-bones version with, for example, nothing between the back of the castle and Hagrid's hut but a field of grass.
In the first two films, Professor Flitwick is an elderly-looking midget◊. From the third onward, he became a small man with brown hair and moustache◊. It was so unexpected that quite a few people joked that he now looked like Hitler. (the story is complicated: as Flitwick wouldn't appear in Prisoner of Azkaban, actor Warwick Davis was instead offered a cameo as the chorus conductor - credited only as "Wizard"; through Retcon, that guy became Flitwick in the fourth movie). Combined with that is the fact that J. K. Rowling wasn't entirely happy with Flitwick's original appearance. The original movie Flitwick looked, in her opinion, a bit like a Goblin, while she had always pictured him as just a very small man.
There's also a drift away from on-location shooting and towards soundstages. At the start of the series, they couldn't afford to build every room in Hogwarts, so there were only a few purpose-built sets and most of the Hogwarts interiors were filmed at various castles, cathedrals, and universities. As the series went along, they built up more and more sets, which was coupled with improvements in CGI technology. Philosopher's Stone was filmed at locations all across Britain, while Deathly Hallows, Part 2 was filmed almost completely at Leavesden Studios. Some places that were originally filmed on location were reproduced as sets later in the series, often accompanied by changes in design — compare the hospital wing in the first movie to the hospital wing in the second movie onwards.
In the first movie, the students wore pointed hats with their uniforms during formal scenes in the Great Hall (you'll recall these hats being tossed in the air when Gryffindor won the House Cup). The hats disappeared in the second film and were never seen again. This is probably due to infrequent mentions of hats being part of the school uniform in the books. Doubtlessly, they were dropped because they looked ridiculous.
In the first film, emphasis is put on the fact that the staircases can change path, and this does have an effect on the plot. In the second and third films, this is relegated to background scenery, and by the fourth they seem to have gone altogether.
The first movie puts a lot more focus on Harry's life with the Dursleys, probably to help get a feel for how much Hogwarts improves him. In the later films, since the audience already understands, they only make up the first ten-fifteen minutes... if they're even present at all.
Also in the first movie, the Dursleys were portrayed as somewhat over-the-top antagonists, with practically no redeeming features who seemed to take almost sadistic pleasure out of torturing Harry. While no more likeable in the later films, the Dursleys (at least the parents, Dudley always remained a bit of a jerk) became a bit more human in their portrayal.
In the third film, when asked if he will sign a form for Harry, Uncle Vernon says he will do it later, if Harry behaves. This happens in the book too, but Harry has to practically threaten Vernon to get him to agree to do it later.
If you watch Dr. No after other James Bond films, it'll be a shock: it's a hard-boiled detective story instead of a spy action thriller - mostly because the budget was low. The fight scenes and car chases are rare and short; the only gadget per se is a mook's Cyanide Pill (Q - here, Major Boothroyd, and not played by Desmond Llewellyn - only appears to change Bond's gun). Even the opening sequence is all wrong. It starts with a series of weird electronic beeps, and the familiar theme doesn't play until Bond shoots the gun barrel, and it starts on the wrong cue (the big dramatic part of the song, instead of the actual intro). Then the barrel wiggles down to the bottom of the screen and the opening scene wipes in from— oh? No, it moves on directly to the opening credits while still playing the Bond tune, over some colorful dots appearing all over the screen. Then it jarringly switches to some upbeat salsa music (not a theme song including the movie's title) over some colorful silhouettes of people dancing for a minute or two, when it again suddenly switches to a salsa rendition of "Three Blind Mice" over the silhouettes of the title mice, which then fade into the actual opening of the movie. To call that opening schizophrenic is being a little too kind to it. In fact, the music-video Bond titles didn't appear in their best-known form until Goldfinger.
Watching Toy Story 15 years after it was originally made, while the composition of the elements is still impressive, it is noticeable how certain textures (hair and fabric) are left rather ambiguous and that the faces of human characters other than Sid are often out of frame. This was due to the technology not yet being at point where it could render organic things realistically: it wasn't until The Incredibles that they took the plunge and made a film about people. The film is also remarkably low-key compared to the bombastic flourishes of later films in the series.
Evil Dead is more a Gorn horror film, rather than the horror comedy of the second. Also none of the other cabin members besides Linda is mentioned in the other films. Ash is far from the Catch Phrase spouting, Bad Ass and Jerk Ass we see in the sequels, instead being a rather bland Final Girl played by a guy. The Necronomicon doesn't have that name and the look of it is completely different from the other films. And lastly, in a subtler example, the Deadites (which aren't named as such until the second movie) are originally just pissed off that the teenagers awoke them from their eons-long sleep, whereas in the sequels they implicitly want to Take Over the World.
Saw. In the first two films, Jigsaw is a brutal Serial Killer with an interesting MO. Also, the first film contains very little gore, and the second only contains a lot of blood, but nothing too explicit beyond that. The Torture Porn that the films became known for didn't really start until the 3rd film, at which point Jigsaw was toned down considerably into a very deranged man with a tragic past, but with his heart in the right place. While still a psychotic villain, he is no longer like the first two films. That role, instead, gets taken over by his apprentices, especially Hoffman.
The original 1984 A Nightmare on Elm Street is much more dark/serious than most of the sequels. Also, Freddy Krueger is identified as "Fred Krueger" in the credits.
Transformers is a more straight-forward story with a relatively short number of robot characters (Sequel Escalation and Serial Escalation were heavy in the two movies that followed). Also, while Bumblebee drops Sam and Mikaela out of his vehicle mode before transforming into his robot mode to fight Barricade, the sequels have the transformations fast enough to safely eject any passengers and quickly convert into robot mode in one quick go.
The first film had Cybertronians bleed blue-green Alien Blood (most likely meant to be Energon). The sequels replace it with a red substance that may or may not be real blood (they're alien robots, remember).
If your knowledge of Twilight comes from Pop-Cultural Osmosis, you'll find the first movie awfully strange. It's essentially a low-budget indie (a very successful one) and it feels like it. There are only the most basic special effects and it generally just feels "small". In contrast, the sequels had higher budgets, so they feel bigger and have a blockbuster "sheen" which the original lacked. Taylor Lautner does appear as Jacob in the first film, but he's just barely in it, never mentions being a werewolf, and (gasp!) keeps his shirt on the whole time.
Jacob not mentioning he's a werewolf is in keeping with the books. In the first book, the existence of werewolves is mentioned but dismissed as a myth, and Jacob is not revealed to be a werewolf until the second book.
The 1954 film Gojira, which kick-started the Godzilla franchise, is a surprisingly dark (and seriously scary) horror film rather than a campy monster movie. Because Godzilla is the only monster appearing in the film, the focus is on the humans' response to his rampage rather than on a battle between opposing monsters. Also, Godzilla is unambiguously presented as a villainous monster incapable of reason or sympathy, and definitely not as a Noble Demon and defender of humanity, as he evolved into as the series went on.
Kitty Pryde (Shadowcat) appears in all three X-Men films, is notably discussed in the US Senate in X1 and referenced by Xavier to the President of the United States in X2, but her on-screen appearances are very brief, almost cameo-sized, and she's played by different actresses in each. In X-Men: The Last Stand, she finally becomes a main character with whole sequences from her perspective, and is portrayed by Ellen Page. The first film also features a different actor as John Allerdyce (Pyro) in a brief cameo. John becomes a main character with a different, lasting actor for the next two films.
Deliberately invoked in spirit in X-Men: First Class, especially if you've seen the other X-men movies, which take place a generation or two later. Among others, there's Xavier acting at times like a cocky, womanizing ditz (compared to his much more subdued and mature persona later on) and Magneto not hesitating to use a gun if he likes to (whereas in the other films he sneers at firearms with disdain).
The first Austin Powers was slightly darker in tone than its sequels, parodied James Bond tropes more directly, had those short segments with Austin dancing to music in-between scenes.
The first Ice Age film featured humans and portrayed all of the animal characters more realistically (they walked like the actual animals always, and could not speak to humankind). In the sequels however, the tone turned Lighter and Softer, all of the humans are written out of existence, and all of the animals are now the main inhabitants of the world.
The first film in The Land Before Time series films is actually the only one that is not a musical. It also has significantly higher animation quality and a much darker tone. Don Bluth, anyone?
The Bourne Identity feels very different in tone to its sequels. Supremacy and "Ultimatum" are contiguous to the point that there is no time-cut at all between the last scenes of the second film and the first scenes of the third film (not counting the Bourne-in-New-York-scene, a Supremacy note which is "also" tied up in Ultimatum...). But when you have recently seen Supremacy and/or Ultimatum, it can come as a bit of a shock to rewatch Identity and realise how different it is, though it was setting up all the Bourne tropes the later films played on. Notably, the soundtrack is a very different beast, employing techno-ish and pop-y background music. Damon's Bourne is also surprisingly chatty and smiley compared to his later silent stoicism. The editing takes a different approach completely, and the camera work is free of the Jitter Cam that defined the sequels. Most of this change in tone has to do with he first film being made by a different director to its sequels, and the fact that there was several years' gap between its release and the release of its first sequel.
Bourne's stoicism throughout the sequels can be explained by the murder of Marie towards the start of Supremacy.
The first Mad Max movie is not postapocalyptic, as are the following two films. The fact that it had No Budget could have been a factor.
The original Poison Ivy was a character-driven thriller with A-Lister Drew Barrymore in the title role, and (here's the kicker) no female nudity at all. The following three sequels were a loosely connected series of erotic dramas starring B-List actresses, known for their gratuitous sex and nudity...and not much else. In an odd subversion, though, the original is probably the most well-known of the four.
The bridge from The Motion Picture is exactly the same bridge used in The Wrath of Khan and The Search for Spock, with the only difference being the lower lighting used in the second and third films.
Star Trek III: The Search for Spock was the first Star Trek production to feature the full Klingon language, and so a lot of the word pronunciations are different to how they would sound in TNG onwards — for example, listen to how Kruge says Qapla' (with a more phonetic sound) just before Torg's boarding party leaves for the Enterprise. The most common explanation for this among fans is that Kruge just has an odd regional accent.
Taking the Batman movies as a whole, the 1966 Batman film was a far cry from Tim Burton's Batman (1989), which has debatably held up quite well, though the the Schumacher films veered closer to that direction. Starting with the 1989 film, the first movie lacked most of the cartoon zaniness for which the '60s film and TV series were ridiculed. For about the first hour, the movie is a serious urban crime drama with the political and economic underpinnings of Gotham City discussed in an adult nature, and the film had a semi-expressionist look to it. Batman doesn't even pretend to be a family film, with plenty of profanity, violence, sexual innuendoes and other adult themes. Especially the characterization of The Joker: even after his transformation, Jack Napier remains for a few more scenes a relatively realistic Mob boss who just happens to look like a clown, and his trademark "gag weapons" are only gradually introduced. This is clearly a "rough cut" of the series' aesthetic as a whole, before Warner Brothers decided to descend to ice-skating henchmen and a Batarang-fetching circus poodle, among other eccentricities.
Some view the films of Joel Schumacher from 1995 and 1997 as the Spiritual Successor to the '60s series, but Batman Forever had its share of serious moments (e.g. the death of Robin's parents).
The four films of the '90s (yeah, Summer of 1989 counts as The Nineties) could, by the same token, be seen as "early installment weirdness" for the trilogy directed by Christopher Nolan, which were intended to be more realistic in terms of style, political underpinnings, etc. Much of this is due to the films being shot on location, as well as a minimum of computer effects.
And to bring it full circle, the serials of the 1940s might be seen this way as none of them had the grandeur (read: budget) of the later films, did not use villains from the comics, familiar Bat-vehicles (e.g. the Batmobile) and like the '60s TV show, tights instead of armor.
The original Teenage Mutant Ninja Turtles movie (1990) is a good deal grittier than its sequels, with an overall more serious tone and fewer comic-book gimmicks. Yes, there is plenty of humor, but it is there only to underscore the brooding nature of the piece rather than to exist for its own sake. Also the jettisoning of most supernatural elements: Master Splinter and the Turtles themselves have a supernatural origin, but their allies and enemies are all ordinary human beings. Not until The Secret of the Ooze (1991) do we get to see used as villains the bizarrely mutated beasts for which the cartoon series and the original comics had become famous.