When speaking about its plot, Our Idiot Brother is your typical indie comedy wherein the fuck-up in the family isn't nearly as fucked up as the 'normal' family members. Predictably, the insertion of this person back into the fold blows up their lives while eventually changing them for the better. With this sort of flick, the plot is largely immaterial*.
What we are left to discuss then is what does and doesn't work. In films like this, the important elements are largely related to cast/characters and the comedy itself. Sure, direction matters, but directorial impact is much less evident in films like Our Idiot Brother. Jesse Peretz's work here is perfectly palatable, and his influence was surely felt in a much more palpable sense than will be given credit here.
By and large, this film works. The lion's share of the burden comes down on the titular character, Paul Rudd's Ned. Luckily, Our Idiot Brother frees Paul Rudd from the shackles of playing the straight man, which he has been stuck playing all too often. Here he is cast in the role of a man who chooses to see the best in everybody and innocently assumes that everybody else does the same, something that routinely gets him into trouble with hilarious results. The supporting cast is filled out by the likes of Steve Coogan, Adam Scott, Elizabeth Banks, Emily Mortimer, Rashida Jones, Zooey Deschanel, Kathryn Hahn, and T.J. Miller. The only potential issue in the supporting cast is Zooey Deschanel, who has either fallen into type-casting hell or really is a one-note actor (in the larger, non-gender defined sense of the word). She isn't called upon to carry too much of the film, as it is a large ensemble cast, and while cuckolding Rashida Jones is indefensible, nothing about her performance here is too grating. With this cast there are great comedy chops at work.
The script, while predictable, shows a love for its characters and is legitimately funny. With this cast and the strong dialogue-/character-driven comedy, Our Idiot Brother may not be a game-changing movie, but it's certainly enjoyable and frees up Paul Rudd to be something other than the straight man.