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.
A meta example, as the movie provides one of its own. Natalie, for the first viewing of the movie comes off as a manipulative, evil bitch, but on the second viewing all of her actions become immensely more justified when you realize that she knew from the beginning the protagonist had murdered her boyfriend.
Another is whether Lenny really killed his wife or it's just another of Teddy's lies is left up to the audience, as there's evidence for each. There's also whether or not Teddy being killed will stop his killing spree. And probably the most poignant one. Is Lenny a victim, being manipulated by Teddy to be a hired gun, who frees himself from Teddy's influence at the end of the movie? Or is Lenny just a serial killer with anterograde amnesia who just wants a purpose in life, even if it means deluding himself? The second has more weight than it initially appears, if one wonders what kind of life Lenny will/can have if he gets away with it.
What about Teddy? Is he just a scheming crook who manipulated a hapless amnesiac into becoming his own personal thug? Or is he, in fact, the only real friend that Lenny has—a devoted pal who's been keeping Lenny out of jail, holding him back from becoming a full-blown serial killer, and giving him purpose and protection—all because he just wants the poor guy to be happy?
Award Snub: Memento was one of the most critically acclaimed films of 2001 and is frequently ranked as one of the best films of the 2000-2009 decade, yet it didn't even receive a Best Picture Oscar nomination.
Christopher Nolan received a Director's Guild Award nomination but was snubbed by the Oscars.
The film did receive two Oscar nominations for Best Original Screenplay and Best Film Editing. However, considering that Memento has appeared on lists of the greatest screenplays and best edited films of all time, one could argue it should have won both nominations.
Leonard arguably crosses this when he decides to consciously forget the truth about what happened and makes it seem as if Teddy was the one who killed his wife, just so he would still feel a sense of purpose. This in itself becomes rather blurry depending on just how you interpret Teddy's character and whether or not what he said was actually true.
The Reveal: Subverted. Teddy hiding his identity as John G is a Foregone Conclusion about a fourth of the way through the film, with the rest of the movie leading up to that point. But he is not the first John G that Leonard has killed.
Rewatch Bonus: The film takes a whole new light when you watch it a second time around. Some of the characters' behavior, such as Natalie's initial attitude with Leonard, makes a lot more sense when you know the details.
Teddy, knowing what he knows about the protagonist's facts concerning the case, and knowing that his own real name is John G., why he went within a hundred miles of this guy can at best be chalked up to greedy recklessness and less charitably just a death wish.
At least he used an alias. Still, it's a bad idea.