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: