Honestly? 25k HP is fine.
Problem is, most people will refuse a tank with anything less then 30k. It's silly, but thats how it is usually (unless you can find willing groups who are also undergeared and unable to get into heroics).
I'd suggest getting Stamina on your wrists instead of Expertise. And at least a Weapon Chain on your mace. And armour patch your legs.
Oh, and max out Mining. That'll help as well.