I would love to talk about level design without it being lost in the shuffle. So many things I'd like to say about the quality of levels I've seen, tips, tricks...
For example, one big thing...think twice about every single bottomless pit/lava section in your level. AFAIK it's the only instant death in the game. I'd imagine pits are the single biggest killer thus far among all levels by all users.
Always think, do I really need this pit? Do I need this section where you bounce on paratroopas or bullet bills for an extended period of time, or even just do some tricky jumps on tiny mushroom platforms? Your level might be much better without it.
I would suggest to everyone that you should challenge yourself to make one or more levels with no pits in them at all, just relying on good enemy placement for difficulty. I don't think you'll regret it.
This is fantastic advice and exactly what I was looking for.