These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Bruno of the Elite Four can be easily taken out by any decent Psychic type. This was true in later generations as well, but somewhat downplayed because: a) Psychic Pokemon were no longer Game Breakers, so players were less likely to have them in their teams; and b) Bruno and other bosses in general wised-up and started utilising other attacking types to counter their weakness.
Creepy Awesome: The infamous Lavender Town theme is absolutely horrifying, and yet so cool at the same time.
Demonic Spiders: Anything that had Wrap or Fire Spin and was faster than your Pokémon was this. All the opponent has to do is use Wrap over and over to prevent you from attacking (AI players have infinite power points, you don't). This was especially bad with Tentacool and Tentacruel, who could couple it with Poison Sting, doing more damage and potentially losing a Pokémon, as well as Erika's Vileplume, who could couple Wrap with Stun Spore to slow you down so Wrap could work indefinitely even if you were initially faster.
Fandom Berserk Button: Do NOT call Red "Ash" on a Pokemon forum. It will not be pretty. Not helped by Nintendo calling the trainer and the rival with the same names from the anime, before they got the names of Red and Green/Blue in Generation II.
X Accuracy in the first games gave every move perfect accuracy. This includes the OHKO instant death moves. The real dangers of this are because beating the eighth Gym Leader Giovanni got you the TM for Fissure which could be used on a lot of final form Pokemon like Dugtrio who could outspeed most Pokemon easily, and because OHKO moves could be used to defeat higher level opponents, it made defeating the Elite Four (barring Lance who could be defeated using a good Ice Beam user, which most Water Pokemon are) a piece of cake. The combination of X Accuracy and OHKO moves was so powerful that the mechanics for OHKO moves had to be changed in future generations so that they could never connect against an opponent whose level was higher than the user's level.
In the first generation of games, the Psychic-type was highest on the elemental tier, as Psychic-types only had a weakness to types of attacks that did below-average damage even after being doubled, and there were very few Pokémon with the stats to use moves of these types anyway (and due to a bug, one of those types actually didn't affect Psychic-type at all, rather than do double damage). And also by the fact that in Generation I, the most common type was the Poison-type (which is weak to Psychic-type), with 33 members, just narrowly beating out the ever-common Water type (32 of the Generation I Pokémon). In later games, the Poison-type Pokémon count fell to the back so far that it was tough to remember why Psychic was ever such a powerful type to begin with. The Psychic-type advantage was made worse by the fact that the stat Special governed both Special Attack and Special Defense. This meant that Pokémon with high Special, such as Psychics, were much more useful than Pokémon with low Special, such as Fighting. This was fixed in later generations by separating them.
Anything that can learn Wrap, Fire Spin and Clamp turned up into this back in the Generation I games, especially because almost all of them have decent speed stats. See Demonic Spiders above for more details. Probably why the moves are nerfed for the future games.
Genius Bonus: The Magikarp line is a reference to a Chinese legend that states that if a carp can jump over the Dragon Gate (believed to be at the top of various waterfalls), it will be transformed into a dragon as a reward. This holds more ground when Pokémon Snap shows a Magikarp jumping into a waterfall and emerging as a Gyarados.
Growing the Beard: Pokemon Yellow actually is a better game than Red and Blue in terms of gameplay, as the majority of bugs, glitches, and imbalances are fixed, and the way is paved for the full beard-growing of the franchise with Gold and Silver.
Good Bad Bugs: Missingno., M, Glitch City, the Safari Zone glitch, and the Mew glitch, and many other bugs can easily be exploited by the player. These games push the Game Boy cartridges to their limit, and the majority of the more famous glitches stem from shortcuts the programmers took in order to be able to fit the game within the relatively tiny amount of memory available. The Missingno. and Glitch City bugs are the result of the games having no actual memory protection, so the game tries to create a Pokémon encounter or environment from the wrong sets of data with no fail safe to notice the difference. As a result, it's possible to break the game to ridiculous extremes and still have it keep on trucking through everything. Additionally, a bug lets Psychic Pokémon be super-effective against Ghost instead of being completely ineffective (all Ghosts in Gen I are also part Poison). This makes Ghost Pokémon rather weak but Psychic Pokémon (like Mewtwo) awesome.
Harsher in Hindsight: In the Pewter City Museum, there is a model of the Space Shuttle Columbia, which disintegrated in 2003. While it's still explicitly referred to as such in the Japanese version of the remakes (since the Japanese versions were released before the accident happened), in the English version, it's simply referred to as "Space Shuttle."
A Lass outside of the Rock Tunnel complains that there should be a pink Pokémon with a floral print. Now that Black and White are out... Given the reported number of unused designs for Pokémon, many which got used later, it could easily have been an In-Joke at the time.
One of the little quirks of the Missingno. glitch is that, due to not having a back sprite, he'll appear as the last pokémon loaded into memory. This later became Zorua and Zoroark'ssignature power - they'll appear as the last pokémon in the team.
It Was His Sled: The existence of Mew, at the time of the game's release, not even Nintendo was aware Mew was programmed into the game.
Sabrina. Psychic types are gods in this generation, so naturally...
Lance, the fourth member of the Elite 4. Four of his Pokémon are Dragon type, which are only vulnerable to Ice and Dragon, and resistant to all fournote Grass, Water, Fire, Electric (in Yellow) starter types. As this is the first (and only) time that particular type is fought in the single-player game, it is entirely likely that the player doesn't have an Ice-type Pokémon or move prepared, nor is there a Dragon-type move that can be used against them — the only Dragon-type move in the game, Dragon Rage, deals a fixed 40 points of damage, and ignores type.
Unfortunate Implications: Upon arriving in the US, Jynx caught some flak for being a blackface caricature (the intention may have been a ganguro instead, but we're not sure). Its skin was changed from black to purple in later games to avoid problems.
YMMVs that apply to FireRed and LeafGreen
Base Breaker: Just like Lyra from HGSS, Leaf suffers from this. Don't get into a conversation on whether or not she exists in the game universe.
Professor Oak's infamous line: "I came when I heard you beat the Elite Four."
Ekans = snake, Arbok = cobra, Muk = ???
Alternate Character Interpretation: In one "interpretation", your rival is the good guy. note In Lavender Town, he asks you "What brings you here? Is your Pokémon dead?" before challenging you, without explaining what he's doing there. You then fight him, and if you've been paying attention, you will notice that he no longer has a Raticate. The theory is that the player character wounded their rival's Raticate so severely that it died of its injuries, spurring him on to beat the Elite Four before the player. But shortly after he has defeated the Elite Four, he must fight the player...who defeats him...and then is congratulated by none other than his grandfather, Professor Oak, who scolds Blue and claims he lost because he did not treat his Pokémon with love and trust. This gives the fairly one-dimensional rival character a deep story and portrays him as a tragic hero with terrible luck. It also makes your character's actions fairly despicable, as all this guy wanted was to be a great Pokémon trainer and win his grandfather's love. Easy to poke holes through this. This does require some big assumptions, though, and doesn't change the fact that he acts like a Jerkass towards you.
Professor Oak has had a few theories devised about him over the years. The first type of theory is that he's a Genius Ditz who understands Pokémon but is too stupid to keep all the data about them (or, indeed, remember his own grandson's name), hence why you have to recollect all of it. The second kind of theory puts him in a somewhat more malevolent light, either as having some illegal ties to the various villain teams or as the Bigger Bad of the franchise.
Pokémon Origins went out of the way to debunk both of these: Blue really is an asshole, but not an irredeemable one. Oak is even called out on sending the two of them out: his response is that both of them have major flaws that this adventure might help them grow out of. He wants Blue to learn compassion and stop being the resident Jerkass and for Red to stop pussyfooting around and Dare to Be Badass. The adventure proves him right: Red becomes one of the most famous badasses in Pokemon history and Blue's defeat at this hand leads to his softening by the time of Pokémon Gold and Silver. History proved him right.
Americans Hate Tingle: Grimer's and Muk's French names of "Tadmorv" and "Grotadmorv", which are condensed versions of "pile of snot" and "big pile of snot", are unpopular with French Canadians. At least one of the two is guaranteed to come up whenever there's a Dub Name Change debate with Quebecers.note This trope applies because the French translations of the games come from France, and weren't even available in Quebec for the longest time.
Germans Love David Hasselhoff: In contrast, Blastoise's French name of "Tortank" is popular with English-speaking fans, having earned special mention on a number of blogs and a Dorkly popularity poll. Since it's derived from "tortue" (which is similar to English "tortoise") and "tank", it can easily pass as a cooler English Pokémon name.note However, the actual French pronunciation is more like "toghr-TONK", not "TOR-tank" (admit it, you probably just read it as the latter).
Dragonite. It's the original pseudo-legendary, and one of the best Dragon-types in the game... that just happens to look like Barney the Dinosaur. The fact that it bears little resemblance to its serpentine pre-evolutions doesn't help matters. Plus, just like Charizard, all the worship it gets from fans who only like the first generation annoys some fans of the newer gens. This may be why they gave Iris' Dragonite a grumpy personality and a generally angry expression in the Black and White anime season, to defy its cuddly appearance and help "unify" its popularity. Incidentally, Red / Blue are the only games in which it is shown with an angry expression, aside from its attack animations in Pokémon X and Y; in fact, every Dragonite card in the TCG from Dragon Vault onwards has shown the Pokémon with a serious expression on its face.
Pikachu, for all of being the series' mascot, sees a lot of abuse both from people tired of seeing Pokémon everywhere, and from fans who feel it gets far too much attention.
Charizard, despite being one of the most popular Pokémon designs in the series for its dragon-like appearance, is regarded in fandom circles as a symbol for "Genwunners" — people who prefer the first generation over any other, and are often perceived as doing little but complain about later games. Essentially becoming the Pokémon equivalent of Wolverine in terms of popularity and exposure doesn't help matters on either end of the spectrum.
Breather Boss: Giovanni is rather easy for being the final Gym Leader, since most of his Pokémon are rather slow and all of them have common weaknesses, or even double weaknesses. Even more so in FireRed and LeafGreen, where his strongest Pokemon in the original Red and Blue — Rhydon — was replaced by its unevolved version, a Rhyhorn, of all things.
Although most of the original 151 Pokémon have become highly regarded in comparison to later additions over time, and a handful having reached superstar levels, at the time of the release of the first generation several Pokémon in particular stood out apart from the starters and Pikachu:
Mewtwo, the strongest Pokémon in the game. It is caught at level 70, has a sleek, alien look to it, and outclasses every other Pokémon in the game in pretty much every way.
Alakazam is one of the strongest Pokémon in the game, as it is Psychic, has an excellent moveset and great stats. It is often replaced by Mewtwo, which is better in almost every way, but by the time the player gets that far, Alakazam will have been in the player's party for most of the game. As a result it is very fondly remembered.
Gengar is a nasty-looking Ghost-type Pokémon with excellent stats and a cool, creepy moveset. As its evolutionary line is the only set of Ghost-type Pokémon in the game, its immunity to Fighting- and Normal-type moves also helped set it apart. This is particularly noteworthy for the originals, as Ghost-type moves didn't work on Psychic Pokémon owing to a case of Good Bad Bugs.
Scyther is a ninja-esque version of a praying mantis, with its scythes being actual metal blades. Thanks to its badass appearance, it quickly became a fan favorite, despite its rarity.
Eevee and its "Eeveelutions" became highly popular for two reasons: first, allowing more than one evolution was a mechanic unique to Eevee at the time, and second, the Eevee-related Pokémon are one and all considered to be adorable.
Gyarados is one of the most physically intimidating Pokémon in the generation, being an enormous, very angry-looking sea dragon. Having come from the incredibly weak Magikarp makes it particularly memorable.
It's probably not a coincidence that all of the above evolutionary lines except the Eeveelutions received Mega Evolutions in Generation VI.
Evil Is Cool: To the dismay of many players, there is no "yes" option when the Rocket grunt at the end of Cerulean Bridge wishes to recruit you into Team Rocket after seeing you battle.
First Installment Wins: The first games, and the original 150 (+1) Pokemon are pretty much synonymous with the series. The starters are always compared to Charmander, Bulbasaur, and Squirtle, the original 150 typically dominate the popularity polls, and Red and Blue/Green/Oak are the most well known protagonist and rival respectively (behind theiranime counterparts, Ash and Gary) and, thanks to the games' Popularity Polynomial compared to the anime, possibly the most popular. Despite all the glitches, general messiness and "Genwunner" backlash, there's little doubt that the originals are the most well-remembered and generally liked among fans both new and old (mostly due to the Pokémon fad at the time of Red/Blue's release, and the fact that it's more common to find fans of newer gens who are accepting of older 'Mons than the other way around).
The fact that Giovanni, the boss of Team Rocket, is also the Viridian City Gym Leader is all but common knowledge among fans of the series who have never played the game. Strangely enough, once you enter the gym, and read the plaque on the statue, it says right there it's Giovanni. And, yet, the character who often greets you at the entrances of the gyms claims he has no idea who the gym leader is!
Your rival beating you to the title of champion and being the True Final Boss after you beat Lance is common knowledge in video game circles.
But it was still news to NintendoCapriSun in his blind LP (well, different generation, but every game follows the same formula.)
The Scrappy: Mr. Mime and Jynx are widely disliked by the fandom for their disturbing designs. Fans of the newer gens also tend to criticize Gen I Pokémon with overly simplistic designs such as Diglett, Magnemite and Grimer (though evolutions introduced in later gens such as Magnezone tend to alleviate this somewhat).
Unintentionally Sympathetic: Sure, Blue may have been a cocky jerk who got what he deserved by losing his title in mere minutes, but having his grandfather come over just to berate him for not caring for his Pokémon? That's cold.
The Woobie: Cubone. You would be too if your mother died after you were born, you couldn't look at the moon because you could see her face in it, and you had to wear her skull to hide your face.