I like this movie the more I watch it....HOWEVER.....
A couple things stick out to me as odd that many others have pointed out:
Bill Hader and Kristen Wigg appear to have wandered in from a totally different film or a nearby SNL skit.
I didn't buy Ryan Reynolds as a low level maintainance guy at a low-rent amusement park for a second. He's....too handsome and clean cut? Totally takes me out of the movie. They should've cast someone else. Which brings me to...
Reynolds looks and acts like he's 36 in this movie and Kristen Stewart looks and acts like she's 15. So the subplot of them hooking up behind his wifes back had an ick factor. (paging Chris Hanson and the To Catch a Preditor crew!)
Also both Jessie and Kristen had the awkward stammer-look down at your feet-act pensive and-uncertain-about-almost-everything-you-say thing down pat. It gets annoying after awhile even though that's the characters they were playing. Kristen Stewart has made a career out of it though.
Oh, and how exactly did he know just where she lived in NYC and how to get there and that she'd be walking back home at the exact moment he got off the bus? In the dark no less?