Hodir has been the most fun fight for me to tank so far in Ulduar. It's chaotic, takes a lot of movement, and is a real threat challenge. I'm only going to talk here about tanking, there's plenty of other good strat guides out there. Tankspot is a good place to start.
Hodir is a single tank fight. The tank needs frost resist: when Hodir gets the Frozen Blows buff he does less melee damage but hits the tank for 40,000 in frost damage every 2.5 seconds. I went ahead and tanked it in a full 3 piece polar set, along with a +20 frost head enchant and a Flask of Chromatic Wonder. Adding a paladin aura the 480 frost resist is probably overkill, but you want well over 200 to mitigate the giant frost hits. Druid tanks seem to be the best choice for this fight, mostly because it's easier to build a resist set if you don't need defence.
For the most part the tank needs to do the same things the rest of the raid is doing. Constantly move (or jump) to avoid Biting Cold stacking up. Stay out of falling icicles. Jump in a snowbank for Flash Freeze. The only real tank crisis moment is Frozen Blows. Not only are you taking monster 40,000 frost hits, but the entire raid is getting nuked hard and your healers are stressed. Help them out with a glyphed Frenzied Regeneration, Barkskin, etc.
The key challenge for the tank in this fight is maintaining threat. It's a mobile fight, and generating threat while moving is not always easy. And the NPCs give a bunch of important DPS buffs to the raid; if all the buffs are used then the casters have 100% haste, double crit damage, and Hodir is taking an extra 50% in spell damage. For short bursts the casters are generating about 5x their usual threat. To keep up the tank needs to take advantage of the NPC buffs, too. In particular the Storm Power crit buff from friendly raid members and the Starlight haste buff are very helpful. Unfortunately bears can't take much advantage of the spell damage debuff on Hodir; a Paladin or DK tank might have an easier time with threat.
I found that with a bit of luck I could get all the buffs on me and then let loose with a Berserk. Once I got that off I was way ahead on threat and no one could catch me. But until that all comes together use every other threat trick you can. Misdirects and Tricks of the Trade help. Even so I was really working to generate enough threat, I even finally worked in Faerie Fire every six seconds as a threat building move (not just a theoretical possibility!) Also keep a close eye on Omen and tell the Hunters, Mages, etc to drop aggro as they can.
Despite what all the guides and DBM say, Hodir enraged on us at exactly 8 minutes in 25 man. Which wiped us at 5%, we'd lost a lot of DPS early. Once your DPS figures out how to avoid most of the damage and the healers can keep everyone alive during Frozen Blows, the fight's in the bag. Don't be afraid to battle rez a key person while tanking, by the way, you get a few seconds during Flash Freeze when it's safe.
The trash on the way to Hodir is fun too. Just to thumb their nose at all us players who whined about AoEing Naxx trash some of the mobs have a special buff on them that protects them against AoE damage. Oh yeah, but the mobs do a nice Blizzard on your party. As my colleague Karthis said
In Soviet Ulduar trash AoEs you!