I've recently gotten my DK up to level 80. He's tank specced, but he also has a DPS offspec.
One thing that confuses me is why Glyph of Disease isn't used for single target. To me, it seems like getting in an extra scourge strike (and saving a GCD that can potentially be used for Death Coil) would be a superior option.
I apologize if this has been asked before. I tried the search function, but it didn't turn up anything relevant.