Don't put anything even remotely referrencing a copyrighted work, for one.
http://kotaku.com/5081159/lbp-levels-getting-moderated-out-of-existence
If I had a PS3, I'd play all you guys' levels.
As much as I'd like to say this is true, the ones that brake the most copyright rules are the msot played. IE the sonic level has like, 2-3 thousand plays and a little over a thousand hearts.
My rules of thumb:
#1 don't give it a generic name, make it something that will catch there attention, if only for a second, so they hover over it and take a second look.
#2 DESCRIPTION. If your levels description says: "There is no description" You'll get very little plays. Just punch in a few sentences.
#3 Use the grid function, people don't like having choppy floors and not quite circle circles with strings hanging awkwardly all over. The cleaner it looks, the better chance of getting hearted.
#4 Try and think outside the box, people are intrigued by things that are different, like my gravity well level.
#5 Pay attention to details. It doesnt matter how awesomly tricky and cool your level is, if it's all made out of the generic cardboard material, ti gets really old really fast. Try using different materials, using the thin setting to make a background, then a different material of the same thickness over top to add colors and shapes to it, IE make a big plate for the background, then stick a bunch of moons of the same thickness but different material inside the same peice, adding a cool effect.
Also, don't be afraid of the stickers. Make sure you get the "master designer" trophy via completing all the tutoriuals (some you need to find by clickign square over an item in your popit menu then clicking tutorial) to get all the hidden stickers.
Theres 2 entire alphabets of stickers, and a whole wack of other neato things you get by finishing the "hidden" tutoriuals.
And finally, #6: Prizes. Add lots of prize bubbles around your level for them to get, space them around and stuff. Try tossing in a couple super prize bubbles for them.
Also, capture some of your special objects you made in your level and put them in the bubbles around, and add in at least a "complete level" prize too.
Also, make sure you update your level when problems are found.
If you want some good examples of making your level looking pretty, hit my character, lionhart180 up, and play chapter 1, and maybe 2, of my elements levels, I put a LOT of time into making them pleasing to the eye. Feel free to copy my ideas ^_^