Looks to me like you don't suck at all...30k unbuffed hp is definately a good start. Take this from the guy who has run H ToC 5 every day for the last month trying to get the Peacekeeper blade to drop (still no luck): ToC is a heal and dps check alot more then it is a tank..at least at your level. If you are a tank running 25k unbuffed and barely defense capped, then you'll have issues. But honestly, in frost presence and with even basic buffs, you'd be sitting at what close to 35-36k health? On top of your planned gloves upgrade, I'd go for the T8 helm.
As far as to why you are taking a lot of incoming damage, I'm not really sure if it's spec related or just how you are dancing your tanking rotation. I don't know a whole lot about deathknight tanking, so I'll leave that to another DK in the audience.
Concerning content you are ready for, you can probably get into an Uld 10 man as the off tank, or even a main tank as long as you aren't in a group that wants to be carried I OT'd Uld 10 at your gear level, and my health was only 37k completely buffed with a stoneblood flask, so I think you would do well in there. You could also stand to do Ony 10...though it might be a little tough on you but once again, it's a heal and dps check, just need a competent group for it.