Tank isn't a hard role to play, but it is the most transparent role. Since what you do affects the party in a more profound way, it makes the mistakes you make seem more glaring then if you were on DPS. That being said, once you understand a fight and its mechanics then generally it becomes a pretty easy thing to do.

BUT, the caveat for tank is that you have to have an awareness of whats going on around you. Mobs turn, tank busters need to be taken, mobs have to be move etc. This becomes second nature with time, but you have to lose the tunnel vision that sometimes comes with being a DPS.