This is all speculation, of course, but i expect to see the following:
I doubt tanks will be able to plow through small houses and the like, when we see some houses get blown up by arty in the PAX vids, which is probably the max destruction they can sustain, there's still a fair bit of house left, so i'm not expecting that.
I do however expect small obstacles will be destroyed, fences, barbed wire, sandbags, maybe smaller tree's and the like, anything you look at and go "a tank should have zero trouble with that" will probably get it's stuff all ruined if you drive into it, the engine should be well capable of that.
But i don't expect any really detailed kind of crushing or breaking, this is an MP game mostly, and thus, destruction has to be a fairly streamlined and simple to avoid using up huuuueg amounts of bandwith just for that, so i expect to see predetermined sections of fences and the like get destroyed, and not only the part you touched.
And lastly, since UE3 can do much more advanced vehical physics, i'll bet the tanks will be more mobile, and able to drive over reasonable obstacles that would have stopped them in the first game, things like short concrete barriers and such.