I can only advise you to read as much about StarCraft II as you can find on the internet. Try multiple gaming sites and especially the official website and all that.
Seriously, I've read a lot about it and I swear, this game will kick ass.
Every of these three episodes will have a single player campaign of about 26-30 missions (they said that on BlizzCon). You will be able to play EVERY race in multiplayer matches in ALL episodes (they confirmed that, too). So, what you get is three EPIC games.
Also: The reason why they plan to release it in episodes simply is they have a HUGE ammount of content. Blizzard always does incredible CGI scenes (and StarCraft II will have more of 'em than any other Blizzar title).
Oh, and you didn't forget the role-playing aspect right?