Well, i'll be the WoW supporter here.
I'm unaware of what Downtime you're all talking about, so far i've played for about 6 months on the US servers and going on 2 for the European servers and none of the servers I chose have faced any serious downtime since the first couple of months in the US where were compensated with free play time. My European server "Sunstrider" only goes down for maintenance. Blizzard continually add more content, new Instances, new class sets, battlegrounds, an honor system, rank specific items, heaps of stuff.
WoW is fun IF you have a group of friends to play with. They make the grind more fun and the quests really easy.
Battlegrounds is amazing, especially Alterac. I've seen it played and it's epic. Warsong Gultch is a little different. It's basically capture the flag. You get put into level groups 21-30, 31-40, 41-50, 51-60 and fight it out to capture the enemies flag 3 times before they capture yours. Basic CTF rules apply, your flag has to be on its peadastal for you to score a point.
You can turn off the General channel also, along with the Yell chat, so that shouldn't be a deciding factor. Still Gastrian has a point, Lamers are abundant. From Ganking to thinking they know everything about Battlegrounds i've met some moronic people in my time in WoW, but having a close group of friends that you can do everything with is really great.
The graphics are specialized, they were created to mimic Warcraft 3's cartoony graphics. The graphics shouldn't be scoffed at lightly though, walking through some of the forests in the game is quite amazing. The particle effects are quite nice also.
Lamer ganking is still abundant after the inclusion of Battlegrounds and the honor system, so be carefull when choosing your server. Make SURE you know what you're getting yourself into if you still want a PvP server, cause it can completely destroy the game for you.
There is a fair bit to do, quests are everywhere, nearly every person that isn't a town guard or service provider (blacksmith, armoursmith) gives you a quest. Once you hit level 60 and are waiting for your Hero Classes (*drool*Demon Hunter*drool*) There's always battlegrounds, or collecting your High level sets which takes forever and when you finally collect it chances are Blizzard are going to release a better set for your class :-p.
They have continued the Warcraft story very well with World of Warcraft, and have left many opertunites open to continue the story, what with the manual saying that both Illidan and Arthas are alive, plotting somewhere in their respective bases.
My experience with others is quite limited to Guild Wars and some other Korean MMO's that aren't really worth playing.
First up, Guild Wars IS NOT an MMORPG, it's a normal RPG with a visual lobby where you can buy items and talk to other players, towns. When actually killing monsters and questing only your group is ported to an isolated area, away from other players, difficulty of these areas is scaled to the level of your party. This is good and bad. Good because you don't have any Ninja looters or mob stealers, bad because the chance of running into someone that can help you with quests is pretty much 0%.
Where Guild Wars excelles is it's awesome Guild Based content. Castle seiges and the like is what i'm talking about. The character class system is quite different to other MMO's. You get to choose 2 character classes, 1 primary and 1 secondary. The primary chooses your look for the class and also your stats, the secondary just gives you extra spells.
Ragnarok: Fun, but no quests at all, so this game is only a grind. That's all there is to do, unless you choose a merchant and then you can sell stuff. Character costimization is limited to head pieces. Class system is different again, you start as a Novice, then you choose your base class, then you get two options to upgrade that base class later on, they continually add more classes up the tree. You can also participate in guild wars.
Lineage II: A beauitful game, absolutely stunning, the graphics are amazing. However, the quests are quite few. They do have a guild wars system, limited to castle sieges. You have to take a castle and protect it from attacks each week, and if you're a leader of a guild you can take a quest to own a wyvern pet that can grow and fly you everwhere. It has an entertaining pet system as well. Quite a fun game. But it has open PvP so expect to get ganked. The game is ever upgrading, it progresses through different chapters of the game, they are currently at chapter 2. This changes the story, quests and purpose of playing so it stays fresh.
Still Lineage wasn't all that fun and didn't urge me to play more and more like what WoW does for me.
My 2cents, hope it helped.