Guild Wars - great for PvP, and somewhat fun for PvE. I like PvE, and I liked GW for about a month & a half of faily soild playing. It's very combat oriented, no non-combat skills, just run out and kill stuff. The quests were ok, but still your average RPG-lite fetch & carry stuff that's just meant to get you to a new area.
Since I don't like PvP, I got bored pretty fast. The PvE part of GW is fun enough, and well laid-out, but it doesn't have the obsessive quality that makes me stick with it. I also didn't like the 'items' that much...they weren't very interesting at the time I played. It has more similarities to DungeonSiege than to Diablo2, IMO, although of course, all 3 are hack-n-slash stuff.
WoW is a lot of fun, even solo...the world is neat, and huge, and fun to explore, but you'll hate all the running around to get from A to B. We're talking 15-20 minute runs or more sometimes. You will get lost and won't know 'what do to do' (quest-wise) many many times. WoW has more stuff to waste great chunks of your time with, like learning the blacksmithing trade etc., so if that's your bag, you'll have great fun with that.
It's hard to get a PUG going in WoW tho, at least early on where everyone likes to go solo most of the time (unless you have a guild to rush you, of course).
The general community of WoW...well, it's Blizzard, you can take a guess.