Back to the Future
Some critics think that Back to the Future was made "hackily" and it didn't drive home what it was trying to do. It wasn't necessarily a sci-fi movie, and it wasn't really a comedy, so it's just stuck in the middle as a movie that doesn't really know what it wants to be. People love it, but mostly for the nostalgia factor.
Jaws
Jaws has recently come under some scrutiny and has even been challenged for a remake in the near future because it failed to adjust to the times. For a two hour movie about sharks, critics think that there should have been more shark action and less delegation about what they were going to do about the shark. It was terrifying for the time, but Jaws doesn't hold up well at all.