I decided to main spec tank for the first time in a few years for this expansion. Really enjoying it so far, up at 633 ilevel currently, although I havent been brave enough to take on challenge modes currently, and my guild and server are pretty under populated so its tough to find groups.
My main question is with DKs supposedly being in such a good position for the 5 mans, due to the resolve mechanics, is it true that we suddenly become the worst tanks in raids due to the same reasons? Obviously with us basically relying on a blood shield as our block, I would imagine we'd be taking more damage overall (even with the DS healing) than other tanks? I dont overly want to spec unholy and dps, but obviously if we're dramatically less viable than other tank specs (apart from maybe bears which are seemingly in a very bad place) it might be tough for me to get a spot on a raid team. I dont want to be a hindrance obviously!
We wont really know the answer to that question until the raids drop. From my feel of things I'd say we're fine to MT, but until the encounters are released and we know how spikey the damage will be, there's just no way to give a good answer to this question. If there are gigantic unpredictable spikes (something blizz said they were trying to phase out) then we'll likely be asked to DPS in place of block tanks.
Re #2, unpredictable spikes won't necessarily favor block tanks. A block tank can experience bad block luck in a given window of time and get gibbed. Encounters with heavy duty sustained physical damage will favor block tanks (unlimited percentage scaling) over DK tanks (limited resolve based scaling). Encounters with *predictable* heavy physical damage might favor DKs with their many cooldowns.
There are lots of knobs for Blizz to turn if they need to adjust DK tanking: mastery, resolve, the shield portion of DS, the heal portion of DS.