I'll just name the films, the reason is all the same : great movies seen first run that we did not expect to be great:

Christmas Story (did not expect it to be this good)
Ferris Bueller <-Looked Cheesy in previews
ROBOCOP <-Blew our MINDS.
SouthPark BLU: Did not expect it to be so good - it's a classic
The Matrix during preview screenings - no idea what it was. A few weeks before publicity started.
Bad Santa - Saw it 5 times in the theater the in one week. The first time I was alone, then next 4 I brought friends and paid for their tickets.
40 Year Old Virgin - No idea it would KILL!
Pootie Tang: It was the weirdest film we ever saw with a crowd.
Pineapple Express preview: Not a great movie, but Judd Apatow was in the lobby and I got to speak with him about Larry Sander Show and Freaks and Geeks.