You will need to have a solid grasp of optimal rotations and how to maximize DPS. Lots of endgame fights have had fairly high DPS checks which require everyone to perform above average for their job. On the other hand, everyone makes mistakes from time to time which may make their rotation get a little wonky.

However, what you aren't realizing is that you'll have tons of practice doing these rotations by the time you reach endgame. It becomes more muscle memory than you would think. Also, the development team has stated that they are going to simplify the jobs to some extent in 4.0. However, no one really knows that that's going to entail.