Why there was no fourth race

WC3 wasn't that imbalanced. It's just it has gotten to the point where WC3 has/had been played so much that the top strategies are there and used often. Also, blizzard isn't balancing the game anymore. Granted I hate the blademaster as much as the next person, but you can only blame people going him so often nowadays because blizzard stopped patching. Overall, WC3's plunge into bland/cookie-cutter strategies is largely a result of them not caring anymore.

I played WC3 (a lot), and I personally felt WC3 was more fun than SC2 and the hero elements helped accompany that.

P.S. You really have to look at how long SC2 was in production and really ask the question (strongly ask) why they didn't include a new race. I just see the final product of SC2 and keep thinking to myself that its really nothing more than a SC1.5. Which isn't a bad thing, I hate SCBW's interface/mechanics more than anyone else. So its nice being able to play SC without being penalized because of a piss poor design (granted it was 1998). I guess my main gripe is that they spent this long developing this game and we really don't have anything new :(