I also noticed your focused on a setup similar to EQ (I also played it quite a bit and even in the past 2 years with the progression servers). The good news is that it isn't as hard coded as it is there. Where if you didn't have a Pure Tank and Pure Healer you couldn't accomplish anything. I was surprised that even in EQ a decently geared paladin or SK prior to Kunark and even into Kunark just couldn't tank as well.
In Wow, you get a lot more options, and there is 6 tanks which are all more than capable of getting the job done, on top of that you can swap them to DPS or Healing if they have those roles.
Additionally stacking the same character has its benefits, similar to EQ where if you stacked Wizards or Mages - it just makes it easier on your gameplay as the pacing is much faster and fluid in Wow.

As you venture into higher level content and raiding - the most important thing is to maintain optimal DPS while handling the mechanics thrown at you. When you have a truly mixed group - their spell and dps rotations are all different. It can be a good and bad thing.

End result, there is a lot to take in - and it's not a bad thing if you hit lvl 120 and realize you want a completely different team or you want to mix it up. Everyone has tons of alts and likes to mix and match. The next expansion (unsure on the time but probably Late fall this year) is cleaning up the leveling process. It should be 50%~ faster than how long it will take right now to get to the same spot. So sticking with it, and not rushing so you don't get burned out. It'll be even easier.