I'm kinda new to the whole DK thing. I've researched a bit here and there for a frost dps spec and found one that works but mainly focuses on super crits. Right now I'm unholy spec and it's freakin awesome. Toward the end of this little post I'll link the specs for what I think works best.
Each tree has an awesome dps and tanking capability. The only differences between the tanking and the dps aspect of the DK are a few select talents at the begining of each with a few here and there further down the trees. But that neither here nor there.
As my current unholy spec I can take on 5-6 mobs 2 lvls higher than what i am and come out either at full health or about half health with full runic power. In my opinion it's the best lvling spec I've found considering that you mainly focus on quest completion and doing it quickly rather than killing everything in your way (Drew). If you want more info on it pst me while I'm on and I can give you reasons for each and the best rotation for dps and survival.
Many people think that the blood tree is the best or second best for tanking, but being able to heal and to do dmg makes survivability a non-issue. I personaly don't enjoy getting hit a bunch and healing myself by slowly killing whatever is in my way. I like the AoE, but here's a great article if you decide to venture into the hard but very rewarding blood spec dps.
I'll post more on frost spec dps when i log back onto my other dk and do some personal testing.
As always, if you ever want me to check something out or if you have question just pst me in game and I'll help you or find out what I can. ^^