Depends on which server you play on and what's happening.
I find almost no lag on the two servers I play on (damn Rodent and his alliance whores) I play on an East Coast (Kargath) server and ping 100~ there. On Doomhammer (which is where I am supposed to be but has too many people) I ping 15~
The only time it lags is when the alliance zergs and there are 200 alliance and 50 horde
Game overall is balanced in PvE... anyone can solo/group and do it well... PvP though, you find definite advantages with races/classes.
You can not speak to Alliance if you are Horde and vice versa. So if you have a friend... be sure to join their side so you can communicate.
Quests - Quests are easy and rewarding at the same time. Thru the course of the game, I'd say questing would equal to about 15 actual levels. so it's very rewarding.
about it being worth 15 bucks... eh, I wouldn't know. I don't pay for my subscription. Friend wanted me to play so he pays for it. (that's the reason I play on a east coast server)... If you can pay 50 cents a day and enjoy it (which I do overall... it's easy to pick up and play) then yes, it's worth it.