Ice Trolls!

It's scary, I know - Different kinds of Trolls. Kinda like different kinds of Justin Bieber, only dangerous, not annoying. Unlike their cousins from Oblivion, these guys are actually dangerous and live in the snowy regions of mighty Skyrim.

Fry dat Ice Troll, mon!

As far as weaknesses go, my logical guess is that this one's weak point matches the Cave Troll's - Fire! Which is great, 'cause who doesn't like breaking out your flamethrower spell and frying themselves some troll meat? No one. None at all.

Equivalents of elementary school aside, in spite of seeing a sole Ice Troll in the released video footage, Todd Howard mentioned that we could run into an entire nest of them, suggesting that they might be pack animals. Not much more to say on this topic, other than we should all look forward to fight them, 'cause it seems exciting!

Cave Trolls!

According to me (don't call me Jim,) the most annoying enemy in Oblivion was the cave / forest troll. It was big and incredibly fast, not to mention hard to kill unless you had a firespell at the ready. I'm pretty sure that it was meant to be scary, but it was mostly annoying.

They do make a return in Skyrim, although seeing them now turns everything around - They're still fast, but not as in the annoying way in Oblivion; It is bigger; It will definitely be hard to kill, I'd say!
We've seen video footage of it's icy counterpart, and it is at least terrifying enough to make Hugh Grant cry - Not that he sets the bar very high.