Under the current mechanics (runes and RP) I don't see a way to make DnD more viable short of (a) consumes less runes, (b) costs nothing, or (c) post patch, make it cost a hair less than the RP that Horn of Winter will generate. The CD of DnD is something else entirely, but I understand that it needs one or it would be rather overpowered in PvP.
That said, having gone through all 10 man content (now working on Sarth +2), the only place I can say I feel I have problems with threat as a DK is on Sarth with drakes. I handle adds for the fight. The biggest problem I run into is when we leave Tenebron up, and I have to be able to pick up both elementals AND whelps when they appear. When we finally managed it I wasn't even AoE specced (Blood - Talent Calculator - World of Warcraft). The hardest part is getting people to understand they can't go all-out AoE on the elementals, because I'm not going to drop DnD on them but pick them up one at a time instead. If I dropped a DnD on the elementals, even with a 15 second CD it's not guaranteed to be up for the whelp spawn, and not being able to pick them up immediately means a dead healer and a wipe.