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.
Alternative Character Interpretation: Done on purpose for the film, where if it was about Stephan and his band trying to make it big but Scott accidentally screws up by being strung on Ramona and they only get successful without him, just like his own ex-girlfriend.
Audience-Alienating Premise: According to this article, that is why the movie didn't do well. The emphasis on the problems of 20-somethings alienated people over 30, the allusions to 80's-90s video games and pop culture alienated most people under the age of 21 (who in most cases wouldn't have been born when the games came out), and Michael Cera has a vocal hatedom.
Probably that song Matthew Patel was singing while he was summoning those demon hipster chicks. In canon, even, it seems.
*Matthew breaks into a Bollywood-style number*
Even though he was a big deal in the graphic novel, the Nega-Scott came completely out of nowhere in the film, except for an incredibly brief moment of probably the most vague foreshadowing you could do. The deleted scenes on the DVD show Scott seeing Nega-Scott's reflection in the mirror twice. So it looks like there was more foreshadowing planned, it just got cut.
Ending Fatigue: Scott goes to Gideon's club, getting past the bouncers with impromptu passwords. He goes inside and makes amends with his band before beating up Gideon's mooks. Then he has to gain the Power of Love and fight Gideon. After that, Knives and Ramona briefly fight so Scott has to break them up and apologize for being a Jerk Ass. Then Gideon comes back and kills him. He goes through Sub-space, learns of Ramona's control chip, and uses the one-up he earned earlier to come back. He then has to do everything all over again. He gets past the bouncers (this time by punching them instead of going through the passwords), fights off the mooks again, makes amends with his band again, gets the Power of Self Respect, defeats Gideon's demon cheerleaders, beats Gideon, stops a fight between Knives and Romana again, apologizes for being a Jerk Ass, frees Ramona from Gideon's control, and the three of them beat Gideon again. [deep breath] Then Gideon sends Nega-Scott after him but that "fight" is at least off-camera. After all that, he goes outside to Knives and Ramona and chooses which one he wants to be with. whew. And all of this takes more than quarter of the whole film.
Hype Backlash: Not a huge amount, but at least a number of people have been vocal about their dislike of the film, mostly out of chagrin towards members of the nerd crowd endlessly going on about it being the best movie ever.
It Gets Better: The movie opens slowly; over a half-hour passes before the first Evil Ex appears, even longer before Gideon enters the picture.
MST3K Mantra: If you're thinking hard about the movie, you're doing it wrong. Just lean back and laugh.
This just applied to the crazy out-of-nowhere Evil Ex fights and (though consistent) unexplained 8/16 bit video game logic. The relationships between the characters are done completely seriously.
Jerkass Woobie: Scott. His jerk traits are a lot more visible in the movie, surprisingly.
It's (arguably) fairly justified for most of what we see since a lot of it stems from the whole Evil Exes thing.
One-Scene Wonder: The Vegan Police. The exes generally have few scenes. In the strictest sense, Matthew Patel and the Katayanagi Twins qualify; we see a short clip from a Lucas Lee film before his one other scene, and Todd Ingram's scenes all lead directly into one another in a ten-minute sequence.
Relationship Writing Fumble: Scott/Knives ended up becoming one in hindsight; originally, the intended ending was for Scott to get back together with Knives, and her increased role in the film was meant to build up to this. Then Edgar Wright learned of the ending to the comic and ended up changing the ending based off of this. So we're left with all this subtext between Scott and Knives without them becoming a couple again.
What an Idiot: Scott gets one from Wallace when Scott gets an email from Matthew Patel, outlining the League of Evil Exes format (Scott dismisses it as rubbish until his fight with Matthew). Todd also gets one when he comments on his second strike by the Vegan Police after his battle with Scott: "Chicken isn't vegan?" This is also a guy who explained the difference between a true vegan and "ovo-lacto-vegetarian":
Scott: Anyone can be vegan... Todd: Ovo-lacto-vegetarian, maybe. Scott: ...ovo-what? Todd: I partake in none of the meat, or the breastmilk, or the ovum, of any creature *dramatic hand gesture* with a face.
Admittedly, Todd was acting like he didn't know to see if he could avert his second strike. Of course, if you see the movie, you'll know if it worked out playing dumb.