If all you're doing is hammering Sunder Armor, you are not a very good tank. I always have plenty to do -- keeping Thunderclap up, keeping Demoralizing Shout up, hitting Shield Block everytime it pops, Revenge everytime it pops, watching the mob casting bar to know when to hit Spell Reflect/Shield Bash, target-switching to tank 2 or more mobs.....all the while watching to make sure nobody's grabbed aggro.
It's still a lot of button pushing, but it's really involved button-pushing. Tanking is probably one most of the most active PVE roles that there is. Switching over to my warlock feels like a vacation.