With the ease of gearing up an 80 tanking paladin, you go from being poorly geared with wonderful healers keeping you alive while you take advantage of all of the various tricks of the trade to practically soloing some of the lower-end heroics. That ability to survive anything also starts to cause your mana to run out. Pulling things at the speed you used to doesn't do enough damage for Spiritual Attunement to do any good, things are dying too fast for Blessing of Sanctuary to be of any help, and every time you hit Consecration, you can physically feel your mana pool get smaller.
While you can pray that you get a druid in your party to hit you with the occasional Innervate, you want to be more self-sufficient, which can cause you to have to change around your tanking style to keep your mana pool filled so that you can actually use your abilities. We'll also take a look at some of the bad ideas on how to handle this.