I'm not by any means an expert, i'm trying hard to find a build i really like for DWing though, but your idea of a build is sort of similar the one i just came up with, the only issue i see with your build (IMO anyway)
i like the general idea but,
A. Merciless Combat is heavy time DPS talent, it sounds nice on paper, but if the mob is at 35% you should have solid threat on it anyway...unless you just want a talent to up your DPS, but in general I wouldn't find that too useful as a tanking talent due to the requirement for 35% and below.
B. I would pick up Rime, the runic power generated from that is crucial IMO, pretty self explanatory
otherwise it doesn't seem to bad and interesting spec imo
http://www.worldofwarcraft.com/info/classes/deathknight/talents.html?tal=0055001403200310000000000003251530100532100301000100000015230000000000000000000000000is the build i was thinking of testing..but the only thing i really do not like about it is lack of GoG. which your build obviously has, i'd really like the additional up time of IBF and the bonus crit damage is always a good thing