1 star: (0 - 5.75 in LDC) These levels need a lot of work. They're totally un-elaborated, and/or inherently flawed due to things like cutoff or enemy spam, or they're untested and there are too many bugs gameplay-wise, in short: Many many things went wrong in such a level.
2 stars: (6 - 9.75) These levels are not terrible, but poor. They're too short, lack scenery or they have errors and/or bugs but it's still barely enjoyable. Again, spend more effort and try to do better. (if it's your first level that I rate two stars, it's not bad. There's a long way you can still go)
3 stars: (10 - 12.75) These levels are about mediocre. They may be still a little bit short, the gameplay is fine but not very original and graphics are also solid, but not breath-taking. Try to keep improving!
4 stars: (13 - 15) These levels are nice, but not the best. Effort has been put into them, there can some professionality be seen in gameplay and graphics, they're a nice experience to play, but there are still things you can do to make it even better. Try to go the extra mile!
5 stars: (15.25 - 20) These are really great levels, those which, as said, I would give more than 15 in an LDC. These suffice many high requirements, and it already goes into subtleties if you want to make them better. These levels are successful all around. Bravo.
These are just general principles I try to follow when rating. Sometimes I don't leave a further comment when rating, most times I do, if you have questions about it, feel free to ask further.
sometime it look fantastic 
