Well seriously I agree with Dawn on this one, it is a game and who cares what others think about how you play, I've played this game for five years now and after about the second year I came to the realization that I could care less about what others think of how I play, that isn't to say I am abnoxious and spam and annoy others when I play, I just do things at my own pace. If I don't want to participate with others I have no qualms about doing so, its my $14.99 (whatever it is) a month and until someone wants to pay my monthly charge, no one is going to tell me how to play.
Also, on the note of not remembering how to play, to be honest, again I 100% agree with Dawn on this one, it is definitely an easy game to (re)pick up and if you find yourself not quite understanding something it really isn't a problem to hop onto one of the many WoW sites out there and read up a little about what is confusing you.
As for feeling overwhelmed I know what you mean both expansions it took me about 6 months to start playing out in the new lands (Outlands and Northrend) when they were first released cause it seemed like a nightmare, but again take your time, do your thing, keep your head low and you'll be fine. I would also seriously recommend finding a smallish guild not a raiding guild at all or even a semi raiding guild and try and jump back into the game like that and once you feel ready look for another guild or group of people that can fulfill your gaming needs.
And most importantly, welcome back!