I am one of the VERY few Blood DK's who are actually attempting HM FL progression, and it SUCKS. I've been one of the best DK tanks on my realm since pre-ICC, and now I'm one of the only ones. Stroll along the Tanking or DK forums, and you see re-rolls or re-specs every single day from the Blood tree to something else, simply because it is entirely too hard to play correctly(Read: what dev's consider fun) while progressing through HM content. Blood DK's have vanished from this world, if you look at progressing guilds.
This playstyle, if transferred to all other tanks in some form or another, will inevitably lead to even less tanks roaming the LFD, Trade chat tanks selling Q's will be a thing of the past, and overall the tank population will go down.
I agree with the populace that tanking has gotten "harder" this x-pac, and the Blood tree is the hardest of them all. If you homogenize the tank specs to be anywhere near similar to Blood in its current state, you will effectively make tanks a thing of the past, and the PvE segment of this game will be unreachable for even more of your customer base.