I'm playing it because there is nothing else interesting to play atm.
First and foremost, Classic is not Vanilla not even close. Classic is much easier to play then vanilla ever was.
There are so many differences that one can not even begin to describe, even in raid encounter mechanics.
- many raid mechanics are different (just one example garr adds do not explode all at once if garr is brought below 50% before they are dead so you can nuke garr simply keeping all adds banished...)
- classic is nerfed to hell and back compared to vanilla, for example some attacks during raid encounters do 90% less damage than in vanilla (300 instead of 3000 for example) so many attacks that one shotted you back in the day you can now just shrug off and continue as nothing much happened
- many other differences
Main issue why its like that is, because Blizzard took retail version of Azeroth world from Legion (which is extremely nerfed compared to vanilla, mechanically and by numbers), and then tried to tweak it back so it's little closer to how it was back then, but there are so many things to tweak and of course they failed to tweak so much of it.
So basically classic, on the first look, looks like vanilla, but it isn't even close to it.
That being said, its a nice game to pass the time while waiting for something more interesting. For a bit of nostalgia, and for social aspects.
I'm playing (alliance) mage and a hunter (there is waaaay too many mages in all servers btw.)
There is waaaaay more horde than alliance so if you want to do BGs, then go for alliance, queue times are MUCH shorter. But if you want easier life in open world, roll a horde.