I'd suggest a jury of maybe 5-7 people. (sth. like 3 of the best players around (morfar; kojak ?) and 2 of the devs)
They should vote on Gameplay and Visuals seperately.
I suggest a voting system where 0 is the lowest and 10 is the highest, and you can choose any integer-value between (incl. 0 and 10;))
Additionally there could be a poll in the forums after the deadline, where everyone can pick his favourite map, and can read the results of the official jury.
The winning map and maybe the follow ups will be a part of the next release.
And i think we all agree on the 2048x2048x2048 rule like posted before, and the map must be GPL'd
Questions that are still open:
How long should the contest last ? (Deadline ?)
-I'm in at any time
When does it start ?
-The sooner the better
Who's in the jury ? (suggestions ?)
and the jury must get the .map-file to look at it to see how well it's mapped (hint brushes etc.)
I don't think that judging the brushwork in-editor is necessary. If it's mapped poorly one will see that ingame and the performance will directly affect the voting on the gameplay-factor. Or we add a third category called "Technical" where fps-drops etc. are taken into account. Of course the *.map file has to come with the *.pk3 if the author wants the map to be included in an upcoming release due to the GPL.