BTW: This game price, or rather: Price for 1/3rd of what Star Craft 2 should be (cause *** Blizaard splitted the game into 3 parts instead of one) is ridiculous. Many better games were priced less... we're paying for the Blizzard logo in intro? What the....
Think of it this way. Starcraft 1 was 30 missions, with three campaigns. SC2: Wings of Liberty is 28 missions, which are not only much more diversified but contain much more plot development and much more polished and well thought out cinematics and such. But it's just one campaign. Heart of the Swarm and Legacy of the Void
will be full price, but they'll also be ~30 missions with only one campaign. With how blown away I am by Wings of Liberty's campaign, I have to think it would be awful if they compressed it all into one game the same size as Starcraft 1. And if they made it larger to accomodate this...
If they did it differently, wouldn't the game still be in development and be less polished overall? (because of people wanting it naow! + no money from releasing the first one)
I'd like to add, though, that with the money from World of Warcrack (11 million strong

) they wouldn't need money from SC1... but moving on.
Will they come with multiplayer improvements/game modes? (Like Dawn of War) Or will it just be a new story?
Yep, just like in SC1: Brood War we'll get new toys to play with as well. So the multiplayer will get more interesting too. Aside from that, you can count on blizzard to maintain the game until 2030 or so when SC3 comes out. Starcraft 1 was still getting patched and rebalanced a decade after its release.
Got another question. Does the Single player require internet connection to run? e.g. can you phsically unplug your ethernet jack, start up single player campaign, and it works?
You have to register it online but then you can play it whenever you want I think. I'm not entirely sure, but you either have the option to "Play as Guest" or sign in to battle.net. If you get disconnected after logging in to BNet you don't get booted out of your single player game though. Found that out thanks to a power surge and a battery backup power strip.
