I have some experience with both games and I do think WOW deserves the monthly fee. I played WOW for about a year and a half before I switched to GW:Factions. I haven't played more than a few days of GW yet but first impressions are that WOW is by far the better game. It is possible this will change and with the limited time spent on GW I can't really make that many useful comparasons between the games. I can however describe my experience with WOW.
Perhaps the fact that WOW was my first MMORPG plays a part but in the beginning I actually felt eager to explore the world in WOW. It felt more lika a real world than GW does for me so far. I'm a quiet and reserved person and would never had asked to be in a guild. It took an hour or so before I was invited to my first guild and I accepted. I have since been in a few guilds and have found people to be generally nice and helpful with a minimum of exceptions.
I have tried most of the different character types availiable and they are different enough to offer plenty of replayability. And most people I have met agree that most of the fun is to be had from level 1-60 (max level). After you cannot level anymore it does turn a bit into trading in for better hats... unless you like PVP, which I don't. That said, while straight up arena PVP isn't my cup of tea, I did participate in several spontanious PVP encounters while grouping in enemy territory and it can be fun at times. The concept of two different sides, horde and alliance, does provide excuses for engaging in spontanious PVP now and then and is a good concept. Now. don't get me wrong. There IS life after level 60. It can still be fun to do instances with good groups and there are several really difficult ones for well organized guilds. But the most fun is while you're still levelling and as I said, there are many characters to try out. (And every character can be customized differently so not every Warrior is the same.)
I like the user interface in WOW and it's very custimizing friendly too. Many people complain about having to "grind". It is true that killing mobs are a large part of WOW and if you do nothing else for hours then sure it can become a bit tedious. Especially if you're unlucky and don't find any good loot for a while. There are certainly quests to do as well. In fact, there are so many quests that it becomes one of the problems with WOW in my opinion. With a limit of 20 active quests the questbook fills up pretty fast and I often felt that it limited my desire to go exploring. What's the point of going to a new place when you can't take any quests there anyway? You can of course terminate a quest and pick it up again later but I didn't like doing that.
Another thing which is both good and bad is that the world isn't as instanced as I have understood that GW is. There are instances of course but for the main part everyone exist in the same world. I like that concept but every now and again there are people, and worse - parties, that just have to be where you are and kill the type of mobs you have a quest killing. As annoying as this can be at times, I still like the concept of everyone being in the same world. (Even if there are several servers of course. It's not like EVE-online where the entire population is on a single server.)
I like the fact that you can, if you so chose, do quite a lot on your own. You don't have to party if you don't feel like it (and sometimes you just don't feel like it) and still feel like you are making progress all the way up to level 60. Then you will probably need to group at some point if you are to keep improving your gear but you can still grind mobs for money to buy stuff at the auction house if you prefer solo play. While not as efficient as grouping and you cannot get the best stuff you can still get pretty far on your own. That said, finding a guild with nice people is half the fun of the game, at least it was for me. And as I stated before, in my experience it isn't that hard to find a friendly guild. And even if you're not in a guild it's not hard to find temporary groups to do quests/instances with.
Well... I guess that's what I can think of right now. Feel free to ask specific questions if you want. I would definately recommend the game. I had a year and a half of fun with it and had it been free I would still be playing it. That said, it is likely that I will return to the game for a while sometime down the line.