I don't actually give any spoilers, just my impression of the single player demo, but for those who want no first impressions, turn aside.
(SPOILER)The single player part of the demo is flat out horrible. It's almost completely action based, and there are parts where you can tell they just took a hatchet to the script when setting up the demo. On the rest, you are praying they took a hatchet to the script, because the intro does not flow smoothly from ME2, and there is a lot of non-interactive scripting.
As for combat, mostly it sucks because of the demo design. They enforced a four gun loadout on characters during the mid-game mission, which cripples you if you aren't a Soldier, since the new weight system means that your abilities reload slower with that many weapons if you don't have the weight to carry them (and, news flash, the other classes don't). Other than that, I just really hate the jump from one cover to the next, mostly because it uses the same commands I always use to exit cover. They bind way, way too many things to the Spacebar (and, since the commands are grouped, you can't freely rebind them).