Broken Base: For many fans of the book and newcomers, the movie is a great adaptation, distilling the unformed and episodic nature of the book and it's many characters into a strong narrative work with the best characters placed to the forefront. However for purist fans of the author, they are uncomfortable with how many of the exciting incidents and dysfunctional characters of the novel were cut out, compressed and simplified. For example, by cutting out Begbie's psycho mates, you lose the various gray shades of psychosis and also make it harder to justify why he's friends with the main group to begin with. The focus on emergent pop music of the 90s and the perception held by some that Danny Boyle and his team of filmmakers suffer from an emphasis of style over substance does not help, leaving this group of fans to consider the film to be like a music video or theme park version of the story. It gets worse when you remember that T2: Trainspotting's main themes revolve around nostalgia, with it chosing to adapt extra scenes from the original novel while diverging further from it's ostensible source, Porno. These fans think it's a case of too little too late, begging the question, why weren't these scenes adapted for the original movie in the first place?
Fans also have issues with, considering how much of the novel and Welsh's wider works bring Leith and Edinburgh to life as a setting, how the film pretty much just shows Princes Street in the first chase scene and then switches to filming in Glasgow. Regardless of how it's easier and economical to film there, this group wished that they had shown more love to the capital of Scotland rather than it's overexposed west coast cousin. Made up for to a good extent by the later Filth and Porno adaptations which benefit from having more scenes shot in Edinburgh.
Cant Un Hear It: Try reading the book with Ewan McGregor's voice as Renton. Or the other castmembers, as the book has multiple narrators. Of course, since the book is written in phonetic Scottish, it actually helps make it easier to understand, especially for Spud and Begbie's chapters.
Helped by the fact that Tam Dean Burn's performances in the audiobooks is quite reminiscent of the actors.
Do Not Do This Cool Thing: Mostly averted. The movie portrays casual drug use as it really is - a series of enjoyable interludes inevitably followed by crashes into depression, sickness, or worse. For the habitual users, their drug fix is a desperate need, and while their lives perhaps remain "fun" by their own definition, they appear squalid, wretched, and disgusting from the perspective of a sober person.
The "Choose life" opening spiel was much imitated in Britain for a while after the film came out, but not as much as the distinctive poster.
The scene where Mark hallucinates Dawn crawling on the ceiling has been parodied a couple of times.
Mexicans Love Speedy Gonzales: Despite the unflattering portrayal of Edinburgh and the "It's shite being Scottish!" monologue, the people of Edinburgh and Scotland, in general, seem to love the movie.
Misaimed Fandom: For a movie with such unsympathetic portrayals of addiction, it has an awful lot of fans in the drug community.
Plenty of real-life opiate addicts are big fans of this movie for several reasons: one, it's about as accurate as movies get in its portrayal of the heroin experience, ups and downs both, and two, it's not overly anvilicious about it in the way that, say, Requiem for a Dream is; it takes more of a neutral, observational stance, rather than outright saying "drugs are bad and everyone who does them will inevitably die horribly" or "hey, drugs are great and nothing bad ever happens to people who do drugs!" It's simply an objective, realistic look at addiction that doesn't try to patronize or hammer a moral into your head.
Begbie has one in some quarters. You're supposed to see him as a violent, empty sociopath. Others see him as Crazy Awesome, especially immature readers/viewers. Welsh himself actually finds him scary to write.
Moral Event Horizon: Begbie's treatment of his girlfriend June. Who is also the mother of his unborn child.
Sick Boy crosses it in Skagboys when he lets the murderer of Maria's father have sex with her. He was planning on killing him, but still...
Nightmare Fuel/Narm: In the movie, there is Renton's dream sequence when his parents lock him in his room to get him off heroin. (The sequence is quite a lot worse in the book. Welsh's writing style is extremely visual and truly horrifying in its description of the hallucinations.) The baby, in particular, is most memorably horrifying. However, the latter may have been a bit too exaggerated to be actually scary, and baby Dawn is clearly an animatronic. It's on at least one list of scariest scenes not in a horror film.
Older Than They Think: The concept of waiters (or waitresses) tainting the food of customers they dislike is probably most closely associated with Fight Club, but this did it first.
The dead baby crawling on the ceiling when Renton goes cold turkey.
Squick: Plenty enough to go around, but most notably when Mrs. Houston enters a tug-of-war with Spud to get her sheets back. The sheets rip open, revealing human feces that sprays everywhere and on everyone in gross detail.
The Woobie: Poor Spud. Other than his self-destructive drug habits, he doesn't do anybody any harm. Yet, he gets sent down to prison while the considerably more deserving Renton escapes it by enrolling in rehab in a quite cynical attempt to make the judge think he's attempting to reform of his own volition. Then, he gets his palm sliced open by Begbie and nearly bleeds out, while his "so-called mates" refuse to call an ambulance for him.
Jerkass Woobie: Tommy, who ends up getting AIDS from using heroin after his girlfriend dumped him (though notably he uses it far less than some of the other addicts who are still in fairly good health). Not only is he dying, but he ends up with hate messages scrawled all over his walls too. His acceptance of his fate and lack of hatred for the guy who gave him that first shot (Renton) makes it in some ways a Heartwarming Moment, but all the more tragic that such a nice guy should end up this way.