Odds are that, at least in the beginning, CCing one or two mobs out of a pack of say 5 is going to be necessary, much like BC. Easiest is to hotkey raidmarks, and throw up a moon for sheep, diamond for sap etc. It's going to mean pulls have to be careful, and recklessly charging in and slamming down thunderclaps won't work. Most of the adjustment is going to be dps who are going to have to be aware of aoe and cc breaking cc, but as the tank you are the group leader by default and it's up to you to judge pulls, do marks, and think about things strategically. It means tanks having to once more take more of an active leadership role, which is nice.
DK tanking is going to change dramatically in Cataclysm, so it's far too early to tell. Blood DK tanks of today and Blood DK tanks of tomorrow are going to be nothing alike. Example, Blood will be getting talents like Bone Shield and Unbreakable Armor moved over, and will lose a lot of dps talents like Hysteria and Dancing Rune Weapon. The rune systems itself is changing as well, so we're talking quite a bit of change and adjustment. It's going to be like playing a new class in many ways. If you want to start tanking as Blood now I'd suggest reading pwnwear.com and the Elitist Jerks DK forum sticky on tanking, but tbh I wouldnt bother starting tanking now, the expac is a couple months away and it brings a clean slate to the DK class.