The last games I can remember playing a demo for were Dragon Age 2 and Mass Effect 3. Of course, we now live in the age of the early access/open beta, so demos are a bit redundant in many cases. The obvious exception, of course, is games developed with entirely in-house QA.
A lot of games only allow early access testing after you've bought the game. It's the real danger zone where especially free test would be really nice even if very low price can mitigate the damages.
The Mt. Hyjal fight at the end of WC3 was that way too, where you aren't supposed to win, just hold on long enough for Malfurion to spring his trap. Kind of interesting the series was bookended like that.
Yeah, I haven't seen a melta/multimelta in the previews yet. Or a Lascannon. Though they are mostly anti-armor, and nids in a hulk mostly aren't that heavily armored.
Never ever ever preorder anything unless you have a lot of unedited footage and there are some REALLY good reasons to do so.
At least you'll be fighting for the Emperor this time, none of that "Dorn was Traitor" bullshit. That will get you blammed for Heresy. Back on topic, I'm tempted by the discount and the sweet facebreaker, but I have to be a bit more conservative with my money this year.