1. Frost Fever no longer debuffs.
On live servers, Frost Fever currently applies a slowing effect. In Mists, it will no longer provide that debuff. The biggest change this brings is that blood tanks may very well find it worthwhile to go diseaseless. On live servers currently, the biggest reason to uses diseases as a tank is for their debuffs. If Frost Fever has no debuff, you are probably better off spending the resources to apply it elsewhere.
Weakened Blows, which reduces physical damage dealt, is still going to be attached to Blood Plague. This gives death knights at least some incentive to use diseases, but here's the thing: A lot of other classes can bring Weakened Blows too. Any tank spec can bring it, as can feral druids, retribution paladins, shaman, hunters (via certain pets), and warriors. If you have a class in your group or raid who can use the debuff and can use it more conveniently than a death knight tank, it becomes a detriment for you to apply diseases.
Blizzard's been unhappy with diseaseless playstyles in the past, so it's unclear if this state of affairs will stay. Of course, on the other hand, there doesn't seem to be an elegant way to force tanks to cast Frost Fever. Adding more threat or damage to it may cause scaling issues.
Personally, my favorite solution would be to just embrace the diseaseless paradigm for blood tanks. Put Weakened Blows on Heart Strike and Blood Boil, and don't worry about requiring us to use diseases.
Of course, the other option is to just give us back the 30-second Outbreak cooldown we've had through the last part of Cataclysm. That cooldown was added for a very specific reason, and while Blizzard took it away in Mists, they haven't fixed the reason we needed it. It is still incredibly detrimental for us to give up a Death Strike to use Plague Strike and Icy Touch. If you can't fix that, give us our cooldown back until you do.
2. There's some downtime to deal with.
As you probably know, DPS death knights won a major victory a few weeks back when Blizzard made the 1.0-second global cooldown a baseline feature of the death knight class. This has been a long-needed change to allow DPS death knights to keep a consistent, smooth damage rotation.
Of course, blood death knights have been getting along just fine with the 1.5-second GCD, and now that we're on the 1-second rotation, it has certainly caused some confusion in that we now have a lot of downtime -- as much as 30%, according to some parses and combat dummy tests.
In theory, this isn't a nerf because we can still do as much as we used to; it just takes less time. Even threat, in theory, wouldn't be a huge issue if you keep Dark Command and Death Grip handy. In practice, though, it can be frustrating to have nothing to do for so long, especially if you're low on health and can't use Death Strike.
Blizzard can fix this, if they choose, a few different ways. They could just give a 1.5-second global cooldown to blood death knights and blood death knights only. Of course, they could also lower the runic power cost of Rune Strike or raise the rune regen granted by Improved Blood Presence, but those could be imbalancing. If they elect not to tweak this, it's good to note that if you start tanking in Mists of Pandaria and seem to have a lot of downtime, it may not be your fault.
3. Blood tank AoE threat has taken a beating.
Another casualty in the Mists of Pandaria balance wars is blood death knight AoE threat. Morbidity and Crimson Scourge are completely gone, and the Glyph of Death and Decay no longer increases the duration of that spell. As a result, blood death knights have fewer options to bolster and sustain their AoE damage and threat. You are most definitely going to have to learn to do more with less, if things stay as they are. Learning to switch between targets to build up threat will likely be a must for anyone who's been putting that off. As you start tanking in Mists, be sure to keep an eye on adds, and if needed, perhaps ask your group to focus on a target so you can more easily keep aggro without being pulled every which way. Single-target threat also took a few hits, as we lost Improved Death Strike as a talent.
If we're lucky, Blizzard might consider throwing us an AoE damage bone such as giving Morbidity back to blood death knights as a spec-specific buff. If not, it's just one more thing to watch out for.
Learn the ropes of endgame play with WoW Insider's DK 101 guide. Make yourself invaluable to your raid group with Mind Freeze and other interrupts, gear up with pre-heroic DPS gear or pre-heroic tank gear, and plot your path to tier 11/valor point DPS gear.