it's kinda strange but as the map pool is getting so big now, it would be really nice if we could vote/rate maps we like. I have a few ideas on how to do this, i think they are all suitable but just thoughts..
First, the trackmania style where you give a map a rating from -3 to 3 (it's written in the form /- /-- /--- /+ /++ /+++). I think this is probably the more complex to enable.
Second, you could use a Thumbs up and/or a thumbs down. This is probably the most straight forward.
Third, a rating which is pretty much the same as the trackmania style but more sensible. Ie, 3 out of 5 or 7 out of 10 (depending on the scale).
The final is probably my favourite which is just a Star. If you like a map, you can give it a star. Starred maps can appear on your profile (maybe a tab that shows your maps | starred maps).
The ability to do this from in-game is a must IMO. So in the console you can type /star or something to star the current map and then display a message to say someone starred it.