I do think that some of the stuff is crappy, but I have also found some great things too. This is what I like at my dollar tree:
pregnancy tests (cheap and they work better than the expensive brands)
puzzles for the kids
large plastic dinosaurs and horses that my kids like to pretend play with
craft supply, their foam stickers are a very good price and cute too
helium balloons for birthdays, gifts, etc.
gift bags (they have the really big ones too, that would cost over $4 at target)
greeting cards (2/$1)
gift wrap (check the sq ft, I try to get the longer length to get more bang for my buck, they have really cute stuff, last time I got some snoopy gift wrap!)
Party favors
Seasonal/party decorations (I have found that generally they are cheaper than a store like factory card outlet)
Books (I've gotten some good deals on books there before)
Hair things (if you have a girl, there are a lot of cute hair things for cheap)
Travel tissue pks (I get these to keep in the car and diaper bag, and they are cheaper than getting it at a regular store, the generic brand is ok with me)