I was a big fan of all Blizzard games until now. Ok, I'm still a big fan.
I finished Warcraft 1,2,3 and Frozen Throne, Diablo 1 & 2 and Diablo 2-LOD, and of course Starcraft and Broodwar.
I played most of them in LAN with friends.
Now, here are my thoughts on Starcraft 2:
# The campaign missions, individually, are fun and sometime difficult (in the higher difficulty levels)
# Overall the campaign scenario is average-bad:
    - It's predictable for the most part
    = Dialogues are sometime fun but most of the time feel like a B or Z action movie (Starship Trooper 2, if you see what I mean) but I think that's on purpose
    + A few funny or cool surprises, including the 
Protoss mini-campaign
    - The end give the feeling 
you've done all this for a trifle (if you played Sc1 & Bw)
    + I can't wait for the expansion to be ready !!!  
 
# The campaign DOESN'T train you for multiplayer, which is a BIG drawback for casual players.
# Multiplayer is good BUT has some stuff I personally don't like:
   *Ok, it's basically 3D Starcraft 1, but with a few things changed, which can be seen as good, or bad.
   *I don't like: 
             - the "macro mechanics": T: Orbital Command Centre (MULE), P: Nexus Chrono Boost  

  , and Z: Queen Larvae Growth
             - why don't I like it ? It's something that distracts from battle, that you HAVE to do, or else you loose (because the opponent will have more money)
             - it has been made for hardcoreans players, to change the game in depth, because it alters the way you upgrade your base etc.
             - no LAN
   *I like
             + Terran: the new building add-ons, the supply depot that can bury, the bunker that can be salvaged
             + Protoss: the gateways, and the phase prism (teleporting units everywhere is the way to go), the colossus ( yay ! )
             + Zerg: the new nydus network, 
             + The graphics and sounds of the game
   *I'm a bit disappointed about:
             = They changed only a few things here and there, and sometime it's just a redesigned unit from Broodwar (Phoenix = Corsair, Corrupter = Devourer)
             = The Zerg: having to manage a queen at each hatchery is so UNZERG. In Starcraft 1 you just selected all your hatcheries and produced massive armies !!
Also: The map editor:
+ you can do anything
- you can't do anything unless you spend HOURS on it