I saw the general level design question but not much of it applies to building good solid levels for RTS games. Where can i find information on rules and mechanics of RTS level design?
Balance (usually symmetry) so that you don't get an unfair advantage by a random spawn point
Allied to this is having a big enough map with enough different start points so that there is an element of FUD at the beginning of the game: 'Where is he?', and the chance to use different strategies such as redeploying bases. It is fine to have a couple of football pitch type maps to learn the game but the whole fog of war/element of surprise is key to RTS games; it rewards reconnaissance and builds suspense.
Defensible positions. Narrow passes, bridges that you can take and hold
Some goal in the no man's land that encourages you to come out of your base to seize the initiative such as taking the high ground, capturing buildings, controlling resources, gaining technology.
Here is an interview with a professional RTS level designer, excerpted from the book Game Level Design By Edward Byrne:
Symmetry is good for balance but I have two things to add:
If you make the different sides of map symmetrical, do this with things that matter for gameplay (such as size and passability of areas) but try to vary the different sides visually where possible (such as different textures and props, using mountains as blockade on one side but buildings on the other). This brings more variety in the map to make it more interesting.
Or to take it further, you could try making maps not symmetrical, while still maintaining the balance by compensating disadvantages with advantages. This is much harder though.
Oh, and sometimes people want to play maps that are fun, and not necessarily balanced at all. It can be interesting to play against a friend "with one hand strapped to your back".
This link is to a German (in English) game designer's blog. He has only done part 1 so far but makes some good points as to how RTS map's should be conceptualized. Leaving some comments will no doubt encourage him to complete the series on RTS map design.
I actually disagree with the 100% even game.
Games such as travian and tribalwars demonstrate a bad example of unevenness. Your position almost completely determines whether you will live or die in the first few weeks.
However if your game is just a team vs team game, which will only go for half an hour to an hour, then you will find that having a randomly generated situation is sometimes quite fun (although it must not be too unbalanced).
The "Pro's" at the game, will be excited by the idea of winning even if it was unfair for them, and when it is easier for them, well big deal, its only half an hour.
The not noobs or pro's people will thrive when advantaged, and learn more when disadvantaged.
And the noobs will enjoy a higher success when it is an easier situation, but will have the excuse of "well i'm still learning" when they get flogged.
This works especially well, when people are matched up to someone of a similar "Level" or "Experience"
Just my 2c