Hello, I came from Alliance side, so I will speak here from Alliance's perspective, though I'm sure some of these will apply to Horde as well
1. I'd say mix of both. Global can be either helpful or cringeworthy at times. These days there are people who looks for 4k+ gs for heroics tho.
2. 2 of my guildies were totally new in WoTLK and both of them caught up just fine. I was also new and now I have 4.2k gs for dps and 3.8k+ for tanking. You will need guild to raid properly though, because most raidings will require achievement
3. World PvP has been gone silent in Icecrown zone these days. I mean I've been dailying and mining there without a single intrusion, (save for one holy pala who stunned me and stealing my titanium). But I think elsewhere are doing just fine. Especially STV, can't stop the pvp there I guess.
As for BGs, I think 80 is active, and for arena, 2s seems active but 3s looks dead.
Horde has been controlling WG for weeks now.
4. All WoTLK raids are supposedly buffed (that means Classic and TBC are buff free) Vault of Archavon and Flame Leviathan are by far the only thing PUGable without much stress. Just a little bit sharing here, but we recently tried PUGing parts of Naxxramas, namely 4 horsemen and Heigan. We did 6 tries on horsemen because tanks kept dying (first and second because of meteor, 3rd and 4th tank died in the middle of tanking because of hard hitting Thane and Baron. 5th randomly 2 shotted). We failed miserably at Heigan. We're 3.6+k gs geared mind you, tanks even more.
5. I think all classes are nice for levelling (prot warrior is faceroll, from personal experience) except maybe rogues. Can't really recommend any classes here, maybe lock and shammy because of some demands?