One other difference that was already sort've mentioned but not expressed:
WoW *feels* massive. If it looks like you can go somewhere, get up on a ledge, climb a hill, chances are you can actually do it. The WoW world *feels* like a real world, where things happen on certain schedules and times during the day, etc.
GW has "invisible walls". The paths aren't linear necessarily, but the paths *are* defined in the game, so you'll come up to "borders" of certain areas and can't cross into others, even though that hill certainly doesn't look that steep. And aside from the cities/outposts and in PvP matches, you don't bump into other players. This is a good and bad thing: good 'cuz of the afore mentioned "no waiting line" or "player sneaking up behind you to still your loot" but bad 'cuz (according to some people) this makes it a little less immersive.
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Also, you never feel like you have to "catch up" and things take less time in GW than they do in WoW.
You never feel like you have to log on everyday, just to keep up with your guildmates or stay competitive with the other players in the game (or feel like you're getting your "money's worth"). Since you're not competing with everybody for loot or kills or whatever, and it's easy to find other players who are at the same level as you, there's not a pressure to "keep up".
Not a high level of "grinding" since there's a level cap and the missions are relatively balanced to increase the challenge as you increase in level. You will find yourself doing some "farming" from time to time in order to build up $$ and crafting materials to get the next level of armor for your class, but that doesn't happen to often, especially if you know in advance what materials you should be saving to get the next set.
And missions can be quick to longish, ranging from 10-30 mins to an hour, so you can play casually when time allows.
Traveling: in WoW, in keeping with the "massiveness", you actually have to spend real time traveling. In GW, once you've reached a city or outpost, you can instantly warp there at anytime. Saves you *a lot* of hassle.
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So, I guess having said all that, GW is actually more like a "massive game" instead of a "massive world". Both WoW and GW have rich stories, histories and narrative, but the game mechanics differ. Many say GW "fixes" the MMORPG genre while others say it really shouldn't be called a MMORPG at all because it diverges a lot from the genre's defining elements.
Personally, I enjoy it because it's balanced, it doesn't eat up a lot of my time, it doesn't cost me anything beyond the initial price, and I can play it at my own pace and feel the need to login everyday. (Of course, the graphics/music/production values are fantastic as well, and the skill set/"character build" game mechanic scratches the old pleasure area of my brain where "Magic The Gathering" used to grasp on so tightly

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