Currently, money can be made via donations and semi purchased content (for example, -Mafia-) and that doesn't seem to motivate people very much.
Anyway, what would successful paid content look like? Not sure there's any situation I think would be worth the hassle. It's a nice thought from Blizzard, but it would be so much more applicable and realistic to just add in game donating, like as a tab under Reviews. And if they really want us to be able to sell content, add a function that'll return how many dollars a player has donated.
Selecting a few key developers/only having new games contain paid content/having professionals make games on an engine that's too complicated for new people and too limited for professionals... It's just seems so impractical.