Welcome to Icecrown! :)
1. Yes, do quests even if they are grey, as long as the exp given isnt too low (remember its a x7 server).
2. I never felt the lvling slowing down, unless being camped by some 80's or something.
3. Im not so sure about the community, since its a new server (Ragnaros + Deathwing). So far, its been great to me, tons of ppl to play with, more active pvp and a fresh start for new people that wants to try wotlk!
The DDoS attacks eventually stop, the *****es that do it eventually get bored of it. Have some patience and enjoy playing!