The first was the Reverse Bear Trap's scene. The series' reputation as the Trope Codifier for Torture Porn is so infamous that few people realize just how light on blood the first film actually was, with many Gory Discretion Shots instead of even a single arterial spray. Jigsaw's death traps were modest in scope, such as being forced to crawl through razor wire, walk barefoot over broken glass, or cut one's foot off in order to escape being locked away forever. The Reverse Bear Trap was among the few exceptions, relying on intricate machinery to tear open the victim's jaw, but even then, it was a small contraption that a skilled engineer like Jigsaw could build in his spare time — and furthermore, the scene ended with the intended victim Amanda escaping from the trap rather than being subjected to its violent punchline. There was also the Drill Chair, but again, not only was the device a comparatively simple one and its intended victim rescued, but it was portrayed as an experimental design on Jigsaw's part.
Overall, the scene didn't factor much into the plot (Amanda's importance came entirely in the sequels), but it was still a standout moment that was featured on one of the film's posters, and so the sequels decided to up the ante. The Sequel Escalation wasn't too bad in Saw II, but by Saw III it had begun to stretch Willing Suspension of Disbelief as to just how a lone nutjob was able to build these overly-complicated clockwork monstrosities that often took up entire rooms, with the Angel Trap that ripped out Kerry's ribcage being the tipping point for many. The new killers taking on the Jigsaw mantle after John's death also weren't engineers like he was (instead being a recovering junkie, a police detective and later on a military veteran), which only strained credibility further, even though John did teach them how to build the traps. Even the justification that Jigsaw's traps got more complex as John gained experience and had apprentices to assist him was effectively nullified with Jigsaw, which revealed that his first multi-victim game, set up alone, had some of the most complex contraptions. Furthermore, the Reverse Bear Trap was the first trap in the series where somebody had to die, as the only way for Amanda to escape was to cut open another person's stomach to retrieve the key. Jigsaw's original motivation (punishing people he deemed to be wasting their lives, but also giving them a chance to survive and redeem themselves) was lost as later films had far more traps that were either inescapable, required one of the participants to kill the other to survive, or left the victims with no agency and required somebody else to save them. The inescapable traps were initially justified by Amanda deviating from the original plans and seeking to outright murder those she judged unworthy, but even this motivation was eventually abandoned, as Amanda and John were both killed off and Hoffman was introduced as a Jigsaw apprentice with his own agenda too. By Saw IV, it was well-established that the reason people saw these movies wasn't to be scared, but rather, to be amazed at what twisted death traps they'd come up with next. It's telling that later films would sometimes go out of their way to have characters die in traps previously left unused or escaped (e.g. Strahm in Saw V by a death room which had one component seen in the previous film, and Jill in Saw 3D by the Reverse Bear Trap).
The second was the film's twist ending. The Reveal that the seemingly dead man in the middle of the room wasn't only still alive, but was in fact Jigsaw didn't really have much of an effect on the plot once you thought about it, especially given the more important reveal in that scene concerning Zep's tape, but it worked at its intended goal of shocking the audience, and when paired with Charlie Clouser's downright epic "Hello Zepp" theme, it became a great moment. The plot twists in the second and third films were better-integrated into their stories, but they also gave the series a reputation for a complex, overarching storyline. Once Lionsgate's executive producers overseeing the series chose to keep the series goingover the wishes of the original creators (who wanted to end the series at the third film), the Myth Arc went from complex to convoluted as new twists and killers were piled on in the sequels, while Jigsaw's original motive was slowly forgotten. Perhaps the increasing levels of gorn were an attempt to compensate for The Chris Carter Effect...
Lastly, Jigsaw won in the first film, in no small part thanks to plans that make less sense the more you think about them and numerous mistakes on the part of his pursuers, and his victims made a good number of bad decisions as well. But the premise was still fresh enough that the audience could remain engaged and overlook some of these, and the film played somewhat fair by allowing Detectives Tapp and Sing to score some victories against Jigsaw by tracking him to his workshop, shooting him, and saving the hostage in the Drill Chair via Cutting the Knot. Adam and Dr. Gordon also made some good decisions like using their shirts to grab the tape recorder and trying to fake Adam's death. Plus, the audience learned Jigsaw's true identity at the end, giving the story some sort of closure. While a Karma Houdini villain in one horror film is a good Sequel Hook, the same villain (plus later Big Bads following his death) constantly evading justice and increasing his victim count to dozens despite being only human (not helped by Jigsaw's successful posthumous schemes, including one against another Big Bad) is obvious Joker Immunity that had to be fueled by constantly adding stupid decisions on the part of his pursuers. As such, Too Bleak, Stopped Caring quickly took hold in many viewers' minds.
Saw II marked the point where the series first began to indulge the Misaimed Fandom and Draco in Leather Pants appeal of Tobin Bell's John Kramer, who has a much larger speaking role than in the first film. The film, through John's expanded origin and relationship with Amanda, established that he was more than just a Serial Killer with a vendetta, gave him a definitive Pet the Dog moment by sparing the teenage Daniel and putting him where the police can easily find him (despite Daniel having presumably failed his game), and emphasized the Asshole Victim nature of the people caught in his traps, thus making him out as a Well-Intentioned ExtremistVigilante Man who makes good points, hurts bad people, always gives them a chance, and always comes out on top. However, at the end of the day he was still presented as a sadistic villain engaged in wildlyDisproportionate Retribution, most of his victims were depicted as A Lighter Shade of Grey who didn't deserve what he put them through no matter what they'd done before, and the idea that what his does isn't murder or that all his victims are guilty is explicitly called out as bullshit by the protagonist Eric. Even the deaths of the most loathsome characters in the film, like Psycho Party Member Xavier, were presented as cases of one evil bastard killing others.
Later films, however, made John's backstory even more tragic with its further extensions, piled on characters who agreed with his philosophy, and shifted focus to his apprentices carrying on his Villainous Legacy, many of whom were more explicitly malicious than John and didn't always allow victims to make a Life-or-Limb Decision to save themselves and learn a harsh moral lesson. Compared to his apprentices, it was now John who was presented as A Lighter Shade of Black as later films emphasized his ostensible moral mission versus the murder sprees of his disciples, even though it was one that had been tinged with hypocrisy on John's part from the start. Even some of the filmmakers bought into the idea that John wasn't a bad guy, and a proposed Heel Realization he'd have had near his death was cut for this reason. The franchise was increasingly stuck with its central character being Unintentionally Unsympathetic, and criticism that the films celebrated their gruesome violence and treated their victims as either deserving it or being improved by it became far more credible.
The second issue Saw II began to raise was its use of Sequencing Deceptions and so-called "apprentice reveals." Much like how the novel idea of revealing people who were working as apprentices or accomplices of Jigsaw became rapidly overused and often poorly used after the film, so did the twist of making seemingly simulatenous storylines turn out to be separate and vice-versa. Saw II was often praised for this in that the "police" and "house" plots were bolstered by Jigsaw's monologues and the trap sequences, respectively, and making the reveal that they weren't happening at the same time was considered pretty creative at the time. Succeeding films, on the other hand, would often have unengaging and uninteresting plots that it seemed like the whole point of the movie was to simply get to the end for at least one twist.
It's worth noting that the three other films that do either the Sequencing Deception, the acolyte reveal, or both — Saw IV, Saw 3D and Jigsaw — are considered to be some of the worst in the series by fans, not only for the films in general but also because of how the twists were pulled. To wit, while Amanda's reveal as a Jigsaw apprentice in II was shown via a stilted monologue on a tape Eric finds alongside a Flashback-Montage Realization, it was helped a lot by the idea that someone would join Jigsaw out of Stockholm Syndrome, and that Jigsaw had eventually succeeded in both his intention of rehabilitating people and his scheme to evade the police; the monologue itself was not excessively long either, and there was plenty of noticeable Foreshadowing to the reveal. By contrast, Hoffman's reveal in IV was not only telegraphed via hints that are very hard to notice, it was also completely unexplained at the time, with little explanation coming before (and said explanation comprising much of the next film's runtime) — though it was quite well-acted by Costas Mandylor, which would help in making Hoffman one of the most beloved characters by the fanbase. The next attempt with Dr. Gordon in 3D was more flawed, as while it was the canonization of a popular fan idea at the time and provided a comeuppance for Hoffman, it had some foreshadowing that was too strong and received little-to-no explanation (even in subsequent films), not helped by Dr. Gordon's attitude throughout the whole thing contrasting greatly with his personality in the first movie. This was repeated one last time with Logan in Jigsaw, which was by far the worst of all — not only was it poorly acted by Matt Passmore, but it had the film's whole plot clarified via a very long Info Dump spoken by Logan in a monotone voice to someone he was going to kill at that moment, on top of still displaying a Flashback-Montage Realization (which Hoffman and Gordon's reveals were fine with solely using in a Show, Don't Tell way) during the whole monologue. In other words, not only was the twists' novelty gone, so too was the strong supporting plot and acting that actually allowed them to land correctly.
To begin with, Saw III was the first entry in the series to adopt the formula of a person undertaking a series of traps in which they themselves generally aren't in any mortal danger, but have to save the lives of people who are somehow connected to them, while learning some sort of life lesson on the way. Here, it made for a new approach after the first two films, in which the protagonists' goal was to simply escape or survive the situation they were in. Many of the future sequels would fall back on this formula (be it entirely or partially in proportion to their runtime) with minimal changes to its basis (at most just playing around with it while prioritizing differences on the surrounding circumstances instead), eventually causing fans to complain that the series had grown stale.
Then there's Jeff's constant inaction and mistakes, which result in the deaths of most of the people he's tasked with saving.note When The Kill Count covered the film, James A. Janisse nicknamed him "Slow-Ass Motherfuckin' Jeff" to mock him, which quickly became a Memetic Mutation among fans of both The Kill Count and Saw. While it doesn't do much to make him a likeable or sympathetic protagonist, it at least makes sense in the context of the film itself, given that Jeff is shown to be a broken-down wreck of a man when the plot starts, and the people in the traps are ones he has a personal grudge against. By contrast, the protagonists in later films are people who should theoretically be competent, and they actually would want to save the victims caught in the traps, making their own similar failures much more glaring.
, the first film in the franchise had two Signature Scenes and an acceptable break from reality that, in hindsight, foreshadowed the problems that plagued the series in its later installments.