I just bought SC2 based around the idea of playing custom maps with Kazz and Det. Playing custom maps in Blizzard RTSs has always been a source of great mirth in previous iterations and while I wanted to wait a while before getting this game due to not liking the demo that much, Kazz and Det playing a "debate" custom map won me over.
Anyway, I dunno if this has been discussed already, but I've really been enjoying the single player campaign due to how it fleshes out the SC universe closer to the degree of Warcraft's.
Gone are the talking heads of the leaders of Terran society and instead we are faced with just a bunch of regular Terran joes trying to survive. People aren't only represented by in battle units anymore and actually have conversations which eachother that develop believable characters.
Raynor's character in SC1 was always there conceptually, but was never presented to the player in an adequate manner. Blizzard's writing staff knew who he was, but players just saw a vulture bike (of all units, christ how impersonal can you get), and then sometimes a green shouldered marine. But now, sitting in Joey's Bar, listening to bluegrass music and lamenting the past with a six shooter next to his drink, we see him with perfect clarity. He's more or less the same character he was before - a western themed Marshal with a dicey past - only now his past is more dicey because it includes the events of the previous game and we can actually appreciate his character and theme.
All of the supporting characters are great too because they all represent a larger concept in the universe and give it a believable human face.
Horner - There are smart, thinking Terrans who don't just blindly follow people like Mensk just because that's who's in charge now. They are people like fleet officers on Battlecruisers.
Swann - In many places there are good working class people who end up being marginalized by all this conquest an war.
Uh, Science lady - Terran technology still progresses due to intelligent and curious minds.
Tosh - Edging out Horner by just a bit, Tosh is my favourite support character and not just because he's the cool, secretive voodoo Ghost character. I like him because he is a psionic powered character who is very superstious. He talks like a protoss who wasn't raised as a protoss so he doesn't know all their terms for things but the same way that they have a doctrine and established religion around their psionic powers, he has superstions and spiritual beliefs. I find this perfectly links all the lore of psionic abilities in Starcrafts universe in a very believable way and helps to draw you into this universe.
Tycus - To me, Tycus is a necessary character to balance out the rest of the cast and does his job brillantly. He represents the only personality the previous game ever actually managed to give off, "hicks in space". He does it in a much cooler manner but the effect is the same. He's there to make sure than even though they have added all these new characters that make the universe better and more believable, this is still Starcraft and hell, it's about time.