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.
Plus, as his own girlfriend points out, if he was that certain of what to do, he'd have taken charge instead of drifting along with what Shaun was doing and making snide comments from the sidelines; he's just a pompous know-it-all when it comes down to it.
A better example might be Shawn's other friend Pete, who is depicted as a bit of an asshole for his obvious contempt for Shaun's best friend Ed, viewing him as a load who holds Shaun back. However, even before the events of the Zombie Apocalypse in which Ed increasingly becomes The Load for real, it's hard not to think that Pete has a point.
Especially ironic, considering the whole joke behind that scene was the implication that Yvonne was the protagonist of a different zombie movie with a higher budget, and that the guys behind Shaun of the Dead could only afford a cameo from the more famous actors. By this point, Wright and Pegg's movies have become popular enough that they actually can afford the A-listers in the big roles, and they're planning on making The World's End a big-scale science fiction epic.
At the end of the movie, there is a brief shot of some talk show featuring a woman who insists of staying with her husband, who is now a zombie. A couple of years later...
Harsherin Hindsight: Shaun's mother does a very convincing zombie impression, even if she didn't mean to. Then you find out that she'd been bitten just moments before.
Some Anvils Need to Be Dropped: No matter what else is going on in your life or how big a rut you're in, making empty promises will sour people's opinions of you and you'll have to work damn hard to regain that trust.
Tear Jerker: A surprising number for what's otherwise a comedy. We have, in order, Phillip's death, before which he tells Shaun that he's always loved him like a son, Barbara turning into a zombie and having to be shot by Shaun, and doomed Ed in the cellar.Mood Whiplash doesn't begin to do some of it justice. In the U.S. at least, it was marketed as a straight-up comedy, which made the Tear Jerker moments come as something of a shock.
What an Idiot: Ed. Deconstructed during the film, though some moments stand out (like having his cell phone on outdoor mode while surrounded by zombies. Being forgetful would at least be understandable, but answering it and holding a conversation wins him the award for dumbest human being ever. Oh, and he's not even chatting to a friend. He's selling weed.)
Roger Ebert:(in his review) When Liz complains that Ed is always around, Shaun says "he doesn't have too many friends," which is often an argument for not becoming one.
David needs to turn off a jukebox to avoid the zombies outside hearing them. However, he can't find the switches, and everyone else is fighting the zombified bartender, so he quickly rushes to the fuse-box. Instead of flipping the switches so that they're all facing the same direction, he starts flipping them all randomly (in sync with the music), resulting in lights turning on and off inside and outside, naturally gaining a lot of attention.
What Do You Mean, It's Not Symbolic?: There's some level of social observation, and the apparent suggestion that in 21st century Britain many people live life in an unthinking, zombie-like state - the introductory footage of everyday life at the film's beginning, Shaun and Ed's first few zombie encounters - in which they fail to notice anything unusual going on, and the ending, where society becomes completely desensitized to violence, and Ed lives on, undead and tied up in the shed. Creepy.
The fact that the first we see of Shaun is that he's shuffling around and yawn/moaning like a zombie sort of stresses the point.