I'm actually not that impressed with the gameplay, mainly because things seem to explode much more rapidly and engagement ranges seem to be much shorter. It's only at the extreme ends of the scale that units seem to be as durable as they were in SC1, and I have my doubts about the Marines actually being so.
This leads to a problem. In SC1's singleplayer, you could solve problems with relatively small numbers of powerful units if you were careful, and the game even encouraged it at times when it carefully overlapped defenses to render smaller units unable to make much of a difference. In SC2's singleplayer, it seems to have been purpose-designed to force a mobbing approach on you, typically using very low-end units. I end up solving all my problems in SC2 with a mixed group of Marauders, Medics, and Marines that's between twenty and thirty-five strong.
I don't even consider Siege Tanks. I might build a couple for base defense but their shortened range makes them offensively unattractive. Vikings are near-useless offensively because they get subjected to both air and ground defenses, so maybe I call down a merc wing for base defense, but typically not. Banshees do come into play in small numbers because they can cloak. Ghosts and nukes are actually boring to use this game, which is a pretty shocking difference compared to the fun I had nuking my way to the Overmind in SC1.