Notes from a talk given at UCI by Dustin Browder on April 18 2012, titled "The Game Design of STARCRAFT II: Designing an E-Sport". You can also see the GDC 2011 video.
For a game to be an e-sport, it needs to be watchable, simple, and clear. It should show player skills, and there should be uncertainty about the outcome. The audience should try to guess the players' next moves.
An e-sport needs few units, so that people don't have to know so much about the game. Plus, with too many units, the expert players figure out which unit is the best for a particular situation. With few units, each unit has more varied types of gameplay; for instance, the Collosus has a strong attack and walks slowly, so it is best used with other units in front of it. However, it also climbs cliffs, so it could be used for taking a base from behind. Similarly, the Force Field spell can be used to protect, split armies, or channel enemies into choke points.
Micro-management is important: that's the R of RTS.
The unit size is important: if Ultralisks or effects were larger than buildings, they would hide zerg rushes, and also make the whole game less clear.
There are standard openings, but there's no standard "5-min building phase" happening in every Starcraft 2 game.
In a "big-army game", units die often, so they can't have a name. Unlike in Warcraft, there's very few heroes. There's also less story.