I completely understand the reasoning behind these changes. DK talents really are a pain in the arse, it requires a lot more investigation into builds then others who just stick to specific talent trees (For the most part anyway). With the passive bonuses, it would just get worse.
But seriously, I'm going to have to relearn both of my specs in Cataclysm, pretty much entirely. I DPS as Blood, which won't be possible, and tank in Frost, which also won't be possible. I know other classes are having major changes done to their talents...but it isn't like being forced out of your preferred specs.
I know people like blood for the self healing, but really, blood is a very effective DPS, as is unholy. Frost...not so much, from what I've seen. Why couldn't they just take the not-so-good Frost and make it a complete tank spec?
Hopefully they can make it up to us blood DPS by showing off new and interesting unholy/frost talents.