This University of Washington research paper (Adobe Acrobat format) attempts to explain the enduring popularity of StarCraft as a spectator support (thanks Ant via Slashdot). The primary conclusion they draw is that the game benefits from information asymmetry, as spectators are aware of things the players aren't, and vice versa. Here's a bit on that:

All information asymmetry is reduced and eliminated as the game progresses. But as the information is revealed, the spectator is entertained in the process. The revelation of this information, slowly teased out, creates suspense for spectators and players. In an article of ―why Starcraft attracts crowds so often‖ listing 14 examples of exciting spectacles [4], the majority were cases of information asymmetry leading to suspense. The examples were: crazy unexpected strategies (player has information advantage), air chases, mine drags, storm blankets, reavers, and the clash (outcomes are highly variable and thus unknown), lurker hold position trick, arbiter recall on mines, and nukes (the spectator has an information advantage over the victim player). For example, as flying transportation vessels float over enemy frontlines, spectators watch in anticipation, wondering if they will be shot down by the opponent‘s turrets or if they will make it to the mining workers, devastating the opponent‘s economy. Starcraft‘s strength in attracting spectators lies in its many situations of long teasing information asymmetries of all three types. These drawn out unknowns build suspense for the spectators and their release is a source of entertainment.