Typhoon Andrew has an article discussing Randomization in PVE Boss fights, mainly quoting Tobold and Craig Stern on the matter.
In my opinion there is too little randomization in MMORPGs. They are nearly totally deterministic. Before combat even starts you know what the monster will do, and what keys to press in which order to optimally defeat it. Thus combat involves no thinking, only execution.
As Craig Stern says, the solution is not making the result of button presses unpredictable, but to make the opponent unpredictable, or the starting situation. That is why card games work: The cards you draw are random, but what you can do with them is not. And in a MMORPG the monsters could be made more unpredictable as well. Why do people need to know in advance what the boss mob is going to do after X minutes to beat him?
Now this whole topic is close to my heart and one of the main reasons why I prefer PVP over PVE. The skill you have available to you and the opposition will not change but what you do with them is always random. You have to think and adjust you strategy based on what your enemy is doing. Yes there will be some degree of predictability to skills i.e. a warrior will use Execute at a certain time etc.
PVE for me is too scripted, I hate the idea of knowing what’s coming, it’s like running on rails, there is no unpredictability. Obviously you don’t want a boss to be completely random in that he uses skills that you were not aware of.
When you fight a warrior you know what you are up against and have a reasonably good idea how to counter him thus you adjust your fight accordingly.
Having a boss fight where he has a known set of skill but uses them randomly would in my view make for a much more interesting fight. It would make people think on there feet.