As a comment on the usefulness of Divine Guardian as a main tank: Several people in this thread have suggested DG is useless for MT-ing. Yes, DG is fairly useless on basic "tank-n-spank" fights, where all you do is stare at a monster's crotch for 5 minutes while spitting out your rotation. However, it's a strong utility spell on technical fights that involve tank-swapping or multiple phases.
Here are some obvious cases where it's quite useful:
- Malygos Deep Breaths - by that point the melee adds should be dead, so you can reduce raid damage considerably here
- Sapphiron Air Phase - I MT Sapphiron every week, and every week I use DG during the air phase, either to cover for a bad blizzard/iceblock combo, or just to reduce blizzard / iceblock damage while getting in position
- Heigan Dance Phase - if you have people with two left feet
- Razuvious, depending on your role in the encounter (I usually tank all the un-controlled trainees, so I don't actually ever get to use it here)
- Gothik, since he swaps tanks. This may only work after the doors open though, never tested it.
- 4H, while traveling between horsemen (either using zerg strategy for the front two, or any time for the back two).
Now admittedly, that's a short list. But it's enough to make DG better than "useless." Also consider that Blizzard has a history of multi-phase technical fights that involve some tank swapping or downtime (c.f. any fight in SWP, most of BT, and assorted fights in TK/SSC/Kara), so it's likely we'll see more in Ulduar.
My only hope is that they make it's replacement off the GCD so i can macro it to my Divine Shield, because a 1-button solution is far more convenient than a 2-button solution.