Jornam and Mozared nailed it pretty well, but playing my own DK tank I feel I oughta chip in for some things they missed.
First of all, specs. Most people use Blood because, well, it works. AoE actually isn't very bad with Blood, DnD, IT+PS, Pest, BB. Depending on the length of the pull, maybe DnD again, or just BB, keeping Pestilences up. You can play any spec you want really, but Blood is the community favorite right now for the very good single target and acceptable AoE, along with simply all of the self-healing you're getting.
DS will get you 15% of your health per hit, then after that it depends on what you talent. Improved Blood Pres. gives your other Pres' the 4% health-as-damage trick. Bloodworms, Rune Tap, etc. Simply great for me because I have more control over if I'm dying or not (Almost solo'd Loken on Heroic from 50%, got him to 5% on my own.) If for some reason your healer isn't working as intended, you can keep yourself alive, in Heroic's anyway. In raids your self healing won't go very far.
As for actual specs, Bladed Armor is a big one. I and most others recommend 5/5. All of that armor gives you really nice AP, and thus more threat through damage. Recommended for all specs.
If you want to stay Frost, as Jornam said, it's a chance to become an RP factory. I did this with great results when I first hit 80. Scent of Blood, as called by others, is simply laughable. I take it on my DK simply because there is nothing else left to take really, the random extra RP is nice sometimes. If you want loads of RP, Chill of the Grave is a must, and then Glyph of Icy Touch!
Elitist Jerks doesn't like this glyph, DPS or tanking, but I found it incredibly useful. That's 25 RP per Icy Touch. Combined with KM, that's alot of crit Frost Strike's and HB's. (Along with the FS glyph, -8 RP cost.) When you use Blood Strike they will become Death Runes, allowing for 4 IT's, leaving you with just 2 Unholy runes, but that wasn't a big deal.
Something I still need to try is something EJ came up with for DPS, called the machine-gun IT spec. You get Death Rune Mastery, in Blood, which requires 13 points, beyond the normally just 10 I would spend there. (Again, for Frost and Unholy, Scent is...not so great.) So, what do you do? Normal for the first rotation, then set it up so you use Pestilence (Depending on if glyphed, if not this is a bit harder), Blood Strike, and 2 Obliterates. End result? 6 Death Rune's. 6 IT's. Loads of Frost Strikes, plus loads of crits from KM. Not so many Rime procs though.
The spec that I used for Frost. (I picked up Morbidity because...eventually...I wanted DnD back on 15 secs CD for easier pulling.) http://www.wowhead.com/?talent#j0EZGxx0AbI0czbuzAo0gh:ksw0VM
Machine-gun IT spec. (Experimental. I'll respec along with someone else on the board if someone wants to test it. Otherwise I'll eventually probably try it and post results eventually. Fill in your own 3rd glyph.) http://www.wowhead.com/?talent#j0E00hZGxx0AbI0czbuzAo0g:ksZ0VM
My Blood spec. (Good threat whilst getting all of the self healing, and Hysteria. Melee DPS will love you for giving it to them.) http://www.wowhead.com/?talent#jcEroI0IosRbosxhxhZ0gh:phimVM