I played my friends copy on his laptop of the first 3 Missions. Now as far as gameplay is concerned, its pretty much the same with improvements in the harvesting system and waypoint system. IE : You can hold shift in Build Mode, allowing you to actually que up multiple buildings at once, allowing the SCV to build one after the other. Also waypoints allow SCVs to auto harvest from min or gas after being built if the waypoint is directed on a gas refinery or min field. (This goes for all races, I just use SCV as a example because I'm so used to playing as Terran) As far as the campaign mode being short, one major reason is because they don't wanna finish it in one game, as there are 2 expansions eventually....(years later......Zzzz) to be released. Now to really get some good challange out of the Campaign, try to limit your tech you use yourself. Especially by not using the Lab Tech Upgrades. (Most of them look pretty helpful in one way or another). Also, if you still feel you want more out of the storyline missions, (though I'm not totally sure on this yet as I dont have full game) you should be able to edit the entire single player missions via Galaxy Edit, and after making it even harder, Add Co-op Mode so you and your friends can go at it.
Now another thing that makes this game unique too is the galaxy editor, which in my mind, is the most powerful map editor for a game ever released thus far, (Except for maybe, and I highly doubt, the Half Life 2 Mod Editor program, which name totally eludes me currently) But yea, This allows you to litterly create infinite game types within itself as the ENTIRE game is able to be modded. Now I may not be a true programmer, but I will be making Co-op campaigns when I get it, and I also seen a lot of fun mods already in the beta of the game (AC-130 Game Type) was a really good one that stood out in my mind. It played like COD MW2 AC-130 Kill Streak Reward, and had you escort a convoy of Terran forces heading toward a base. Outside of that, Ive seen Metal Slug remake in it, along with a start of a LOT of RPG gametypes. So yea, its definitely worth checking out, if just for the galaxy editor and story mode alone. I admit to not being all that good at competitive gameplay, but We all know Blizzard normally (hopefully there fame wont go to there heads like EA or other companies) keeps patching the game to balance it out after receiving player feedback.
Now, on all of these notes, I will say, the first mission didnt impress me much, but there really wasnt much to it. But there was more to it then the first mission in SC1. Because of all the years I slaved within SC1 campaigns I feel like its improved, and I'm sure once I get it, I wont regret it.
If after a million reviews and gameplay videos online still don't sway you into getting it. Well, See if any of your friends have it and check it out there, at least then you can know for sure if you want it or not.