This video discusses why de_dust2, a Counter Strike asymmetrical map, is successful.
Points that were raised in the video:
Background colors contrasting with the players help make them more easily seen at a distance. (The opposite is true as well - similar colors helps camouflage players.)
Having a forced single path that you must always traverse can lead to boredom and wasted game play. It isn't interesting because you have no choices to make - you must take that path. de_dust2 on the other hand provides choices, and then more choices. But it also doesn't offer too many choices that makes the game play overly complicated.
Having too many ways into a base is difficult to defend. Having just the right number allows for pushing out the defense line.
Sniper positions overlooking a main contestable area allows for intel gathering by the defense, and forces the offense to play the snipers.
Sniper positions or any forward position should not take defenders too far from the objective that they need to defend (e.g., flag in their base).
Spawning should allow for minimal effort to any of multiple objectives when defending.