Well, being a long time TA fan, I've been quite disapointed by SC demo. I still haven't tried the skirmish mod, but the little I saw while doing the first campaign map was not that great gameplay wise.

First, there are way to much things on the interface. While TA used to only have a few buttons here and there, SC is fullfilled with "toggle this-toggle that" buttons. The thing is, from what I remember, there's no "On / Off" indication, meaning that you basically don't know what is actually toggle-ed and what is not, and that you'll likely end up with a crapload of stuff on the screen (units' range, radar range, etc.)
Some unit have 2 kind of attacks (ie. against vehicle or against planes). You can toggle from one attack to another by clicking a button, but once again, either I'm blind, either there's no indication as to what kind of attack is selected.

Overall, what I found the most disturbing is the scale :
first, the units and buildings are supposed to be huge. Basically, it means that the map looks quite empty and blank. The first campaign map is really boring. Trees and civil buildings are not even half the size of the crappiest-smallest unit available. If you don't zoom in, forests will look like some green vomite on the screen. Only one or two mountains there and there, a few vomite-like forests, a few destroyed buildings, and that's about it. But then, the first mission apparently takes place in a desert. Let's hope other maps are not as blank.
Secondly, the fact you always have to side loads and loads of (often huge) units, added to the fact there's no minimap means that you constantly have to zoom out to as far as possible to only have a glimpse of what is happening currently.

I'm sure I'll get used to this new gameplay, but I'd have hoped for something as simple as good ol' TA.