I've gotten burned like twice out of all the apps I've bought which is quite a few. I don't do all that much, but here it is.
Lite versions first and foremost; always helpful.
Gameplay videos.
Professional reviews. (pocketgamer.uk and appvee, both have iphone apps that are free. I suggest downloading them. One note though; I find Appvee to be fairly consistently too critical.)
I read negative user reviews in the app store. If they're 1 star and give good reasons, I take those into account. I don't believe the overall score unless there's a lot of reviews because some people give bad ratings and others think 1 star is best, which screws up apps with less reviews.
The publisher/developer matters too. If it's EA I expect a more thorough game, but not necessarily innovative because they're a big game churning giant. NGMoco is pretty creative, Gameloft does some pretty stunning stuff, and I really trust Chillingo as a publisher who supports great games.
Hope it helps!
Nate