The thing I always forget is: Play test your levels to make sure people can beat them... I submitted two impossible levels to McAleead at LU... And my first LE packs had enough back-routes to make two entire new packs with just back-routes...
Also, there are two styles of LemEdit levels -- the first starts with an idea for a level, and places blocks accordingly to fit the idea. This is best used, well... when you have a level idea. The other way is used best if you don't HAVE an idea for a level -- just start placing terrain blocks in a pattern that looks good, then run the level with many skills, and come up with a level idea from that. Adjust the terrain and skills given as necessary. My later level packs have a LOT of levels like this.