Er... DKs are only good at later levels really. They're like pallys with a bit more damage... skills are more focused on "attacking" rather than "defending" as in the case of the pally.
Harmtouch (level 10) is a good way to automatically gain the upper hand in battle (35dmg!) but has a stupid 15 min cd.
BecomeDeath (idk what level, maybe 15?) makes most hostile mobs passvie to you (good for night traveling) and allows for underwater breathing (your breath bar doesn't go down!). It lasts for a loooooong time too.
Curse (level 40) is somewhat broken, sort of. It's ok compared to bladegrasp, but the annoying thing is you have to target a player. Carry around a stack of books and you don't have to worry about reagents. I think at higher levels comes a higher chance that they will miss.
Curse also, unlike pally "invuln", affects your target, not you. Therefore it could also help your buddies.
Decay (level 45) I imagine is a poison skill, I got 4 more levels for that
Drainsoul (level 50) is another direct dmg skill, deals 20 dmg, only it gives you 10 hp (at least that's what the wiki says).
All in all, if you want to have good armor but not have shit dmg (pally does 5 dmg w/ dia sword... compared to DK doing 6 dmg w/ dia axe), then I'd recommend DK.
However in my experience DKs are best in a group, despite having high hp a 1v1 with a samurai/ninja/BM/other dmg dealing class may prove fatal. It's certainly possible to beat them though. I was at the north road one time and was running circles around shrimpy77, and he wasn't hitting me.
I wish you best of luck