Oh my, I asked a question. It's like I actually care about what all three of my readers think! It's no surprise that I write more for my own amusement than anyone else's. You know when the guild forums are slow and you just really need to have a good wordgasm before your head explodes? No? Good, me neither; moving on.
Working with the increasingly more accurate assumption that all classes are as good at filling their potential roles as any other, it becomes clear that the most important choice after deciding whether you want to tank, heal or DPS is how you want to do it. DPS is most obviously grouped into ranged and melee both with wildly different feels, but after that it's just the little things that make the difference.
Lets take my favorite class as an example, druid obviously. You can do whatever you want right off the bat, that's a great start. I originally chose to make a druid for that reason along and also because changing forms looked really fun. Little did I know that it would take me a year and a day to actually get any of those forms when first starting out. I actually made a hunter and got that to max level first just because druids are such late bloomers. Had I known that at the time I probably would have stuck with my druid.
It wasn't until I got cat form that I really started to enjoy my druid at that used to be at level 20. I like instants, cast times make me sleepy, so most all casters don't really hold my attention long. So 20 levels of casting spells that I wasn't even taking talents for was pretty lame. Glad they fixed that in cataclysm. Sadly cataclysm also took away my druid's ability to heal nearly exclusively with instants. I just had a wild idea of all druid healing spells being instant with different mana costs and cooldowns. That'd be sweet.
What I didn't know about druids when I started, but really appreciate now is that lack of (self)buffs to maintain. Paladins have an aura and a seal and a buff, rogues have three different weapons to poison, shaman have weapon enhancements and the list goes on. Who want's to keep track of all that crap? It's not like you get to do perform any better than classes that don't. Druids have their one buff which lasts an hour and if you are lucky you can even pawn it off on a pally or another druid (there always seems to be a plethora of them in some combination). Having a buff to watch/maintain as part of a rotation makes things interesting. Having a buff that you never think about because it's on for an hour or more without any action on your part is just stupid.
At the other extreme there is the abilities that aren't annoying because they are easy to forget about, but because they are like a blinking idiot light on your dashboard. Ooh, ooh, hit this button, now this button, now this other one again. It's probably mostly my personal preference, but I don't enjoy playing whack-a-mole with my action bars. I like a sense of priority between abilities and cooldowns or dots, basically something that you can see coming and anticipate. Obviously I abhor paladins right now because thier action bars light up like a Christmas tree to some pagan RNG holiday. There is enough of that in boss encounters, don't need to add to it. Again, why deal with more hassle when it doesn't make you any better than any other class? I used to refer to these types of abilities as things that needed babysitting as you had to continually watch them to see if they would need your attention. Nowadays of course they light up quite obviously so there is less attention needed, but I still find it equally annoying.
Last but now least I look for flexibility, not to be confused with the versatility of being able to fill a number of different roles, but being able to fill one role well in different situations. We all know the distinction between ranged friendly and melee friendly fights. I love to be able to cross that barrier and perform well on both sides of the fence. Feral druids do that in a couple ways when we can't always be in melee range: we can make sure to keep bleeds refreshed to last while we are away and we have an energy resource that allows us to buffer attacks for a few seconds without losing anything but a few auto attacks.
So I look for versatility, instants, lack of busy work, lack of babysitting, a lot of utility and last but not least flexibility.
The thing that got me thinking of all this to begin with was AOE though. Specifically my shaman and other classes compared to my druid. Shaman have to put up flame shock and hit fire nova, plus totems and if your elemental earthquake, that's a lot of crap to do to compared to a feral druid who hits one button and does better than you with a totally minimal effort. Even balance druids have those stupid mushrooms on top of their previously equally trivial hurricane so not even all druids have it easy. Aside from a channeled blanket AOE or spammable button everyone else is just doing more work than everyone else for absolutely no gain. Class homogenization is often looked upon as a really bad thing, but I think I just found his evil twin brother. There has to be some sort of happy medium where the effort required remains constant while preserving the uniqueness of each class, channeled vs instant burst vs AOE damage over time abilities pretty much do that now.