If you've submitted a map and thought you deserved higher, well tough luck because the score is not going to change. You can request that your map is taken off the site, but come on, is it really worth it?
Basically the ratings come from my (StevieB's) personal opinion, which means the maps must be clean and free of glitches to get a good score. Any floating buildings or half buried objects will be noted. Obviously playability has its say to, it's no good if people can't find each other, I want to see action.
A brief summary of the scores can be seen below:

1/10 - Were you dead when you made this
2/10 - Steaming pile of crap
3/10 - LMAO
4/10 - Poor
5/10 - Seen a lot better
6/10 - Regular Map
7/10 - Nice map, that's more like it
8/10 - Very good map
9/10 - Amazing map, download now
10/10 - LOL, you wish