I don't know about films I loved at first, but I now think are "bad" per se, but there are a lot of sci-fi/fantasy/superhero action films from the last decade or so that have not aged well for me.
Avatar is the big one. Loved it when I first saw it, even though the manipulative storytelling annoyed me even from the first viewing, but after I got it on BluRay, I watched it once, and then never again. I don't hate it like a lot of people; it's a decent enough flick, it just didn't have as much replay value for me as I had hoped.
Watchmen is another. I thought it was great when I first saw it, but over time, the pacing issues and Zak Snyder's obnoxious directing really got on my nerves. Now I can't enjoy it at all aside from a few highlights (like the scenes with Rorschach in prison).
Most Marvel movies fall into this category for me as well. Fun the first time, but not so great when revisited. I used to defend some of these seriously flawed movies (like the first Wolverine in particular) because I didn't want to admit how bad they were, but after a while, I realized I was lying to myself. A lot of these Marvel films are just barely passable schlock at best. There are a few of them (Guardians of the Galaxy, the first Avengers, the last couple X-Men movies) that are genuinely good films, but most of them I could die happy without ever seeing again.
Also, I would like to add that I, too, thought the Hangover aged like crap. I thought it was hilarious the first time I saw it (although I never thought it was "genius"; I remember thinking at the time that even Hot Tub Time Machine had a better written script, and still do), but nowadays it can barely manage to get me to crack a smile. It's not bad exactly, just not really all that great. The sequels, OTOH, I thought were crap from the beginning, and my opinions on those have remained the same ever since.