What 'Elements' -make a great level?

[AdrenalinDragon]

From what I've gathered over the last few weeks, rockdet pretty much has nailed it. The best overall levels are the ones that can be solved without too much trial and error, and ways in which a player can avoid a mistake if he/she doesn't rush. Longer levels with harsh ending parts (You Can't Teach An Old Frog New Tricks and Final Destination) are fine for me, as long as they don't rely on total luck and players can eventually work it out without too much luck involved.