There's no real suspense.
The plot moves at a glacial pace.
The characters do stupid things for no good reasons.
Instead of caring about the main characters, you just kind of feel soiled and embarrassed by them, and vaguely apprehensive about what mistakes they'll make next. Hey, for the exact same experience, just head down to the video store and rent:
At least you'll have Michelle Pfeiffer to look at...
Mark W.
Hey, for the exact same experience, just head down to the video store and rent What Lies Beneath.
At least you'll have Michelle Pfeiffer to look at... Diane Lane's not bad to look at either...
I thought "What Lies Beneath" was an okay movie. The plot had some real suspense, and was purposeful (albeit slow-paced). Harrison Ford was able to convey at least a semi-plausible motivation for his character's actions.
So if you hated "What Lies Beneath" and I thought it was okay, but I hated "Unfaithful", that should tell you something about the latter's relative merits.