The OC (I think season 5 could have redeemed the show a little bit)
Pushing Daisies
Death Valley
LOST wasn't actually "cancelled". I think the writers wanted to end it. It was time. Love this show more than anything, but i'm glad it ended when it did.
I'm not sure whether it could have managed another season, but it should definitely have been given a full final season. Cutting it in half was unfair.
The O.C. was a show that could have had 5 seasons, but as annoying as Marissa was...by killing her off they did weaken the cast and show. They should have kept her on for another season. I'm not sure how good another season would have been the way it was, it got silly and people didn't seem to like the characters as much.