Lack of checkpoints is probably the worst. I'm kinda narked every time I play a level that's someone's first, experimental, single-screen level, containing only a start and an end point, and a couple of clouds.
I dont like 1SLs either! They are a waste of time to me. They are either empty or too cluttered. Like, is it too hard to branch off of one screen?
Hey southern man. I tried playing some of your SS levels last weekend but they said they weren't dl'd correctly. Just a heads up. I did get on one of them I think. Not bad. Keep up the level design stuff man!