Why didn't Annie check out Troy and Abed's apartment properly before moving in? At the very least she should have inspected the room she thought she would be living in.
Abed, of all people, should have been Genre Savvy enough to know that Annie would not be happy living in a blanket fort. Didn't he watch any movies that dealt with a similar situation?
There aren't, to be fair, a heck of a lot of movies or TV shows where a pair of room-mates have a disagreement over one of them being forced to sleep in a blanket-fort in lieu of another bedroom.
Both Abed and Troy seem to grab this actually. While they have always been Man Children, it seems much more exaggerated in this episode.
In keeping with the note under Deconstruction, however, the point of the episode is that living with a pair of Adult Children, no matter how comparatively minor, can get infuriating fast if they're not willing to tone things down a bit.
It has been stated in a previous episode that spending more than five minutes with Abed can get annoying pretty fast.
Why would Troy and Abed put Annie in the blanket fort room rather than use it themselves to begin with? Actually, why would Abed ever even pick the room that doesn't have a TV? Or indeed a blanket fort?
It may have been their misguided plan to be considerate to Annie, giving her what they believed to be the better bedroom. Of course, this reasoning in itself is an Idiot Ball.
Why would Jeff spend the whole day with Dean Pelton, someone who greatly annoys him and is obviously a Stalker with a Crush, instead of just admitting the truth to the group? Sure they might get angry at him, but ditching out on helping a friend move is a relatively minor misdeed compared to some of his other acts.
Because Jeff is the guy who always takes the easy way out of any given situation, even if it's ultimately the worst choice — and at that point, it looks like the easiest way out of that particular situation is to just suck it up and spend a bit of time hanging out with Dean Pelton (especially since he's already there) rather than go to the group, 'fess up and take the aggravation and drama, however ultimately minimal, the group will most likely send his way. When he agrees, he figures it'll just be a bit of shopping and maybe dinner — he doesn't expect karaoke.
Moving this here; it's not a YMMV trope but it seems to be a bit of a thread mode magnet, and also sees a bit subjective at least.