What constitutes good level design?

[brunoafh]

It forces you to learn through playing how to control the game's mechanics

This is by far one of the most important and abandoned aspects of level design. A great point.

[TheFuzz]

I like games where you don't lose a ton of progress from dying. Nothing's more frustrating than to spend an hour trying to get past one point in a level, losing a lot of life doing it, and having your perseverance rewarded by dying at a boss that you didn't have the life to beat, and having to completely redo that part it took you so long to get past. I guess just having frequent checkpoints (as has been mentioned), and putting them after parts that a player will have trouble getting through.

[GigaPepsiMan]

To me a level has to have somes of these things before it can be considered good:
A map that is easy to follow but not too linear
A chance for exploration that does make you too side tracked or lost
Should try to look nice.

Things that you want to avoid:
confusion.
pointless stuff.
Grey.
Annoying death traps that are there not to add challange but to kill you.