Well, depends on the fight. There are a number of fights where there is exactly one way to do it and it never really varies and no one really reacts to a whole lot. (Patchwerk, Chimaeron, etc)Execution vs Depth. WoW fights usually don't have too much depth, since the fights have a set of very strict mechanics, that leads to specific tactics needing to be executed. WoW fights becomes more and more about execution, at least compared to Vanilla and GW2.
On the other hand, there are a lot of fights that are still chaotic even with the execution-demanding mechanics involved. Many of these fights - in contrast to the ones I mentioned above - are fights that most players seem to agree were both entertaining and challenging. (Damn near every heroic Tier 11 fight except...Chimaeron and Atramedes? H-Lich King, H-Anub'Arak. Yes, I'm using really recent examples! Here, Razorgore! There's an older one.)While hard hitting bosses with tons of health might seem like a lazy way to make boss fights, it does mean you can get away with being a little more chaotic than in your everyday WoW raid.
Or put another way, execution and "chaotic" aren't mutually exclusive concepts. A good PvE developer shouldn't feel the need to abandon mechanics and simply stack on numbers (and thus, gear checks) to make content interesting.