Last time around, Massively introduced us WoW players to Age of Conan, and today they've posted a guide to Guild Wars for us World of Warcraft players. Unlike AoC, I have actually played Guild Wars, and they've hit the nail on the head here: for two of the most popular MMOs, Guild Wars is very different from our experience in Azeroth. From a completely instanced overworld (everything outside the cities is instanced, so you won't see anyone else out there questing with you unless they're in your group) to a much more streamlined leveling system and a completely different PvP game (you actually create different characters for PvP, something WoW players have actually been asking for), Guild Wars is a very different game, and definitely an experience worth trying for those of us ingrained with the way they do things in Azeroth.
However, one thing Massively missed was the community of Guild Wars -- because the game doesn't have a monthly fee at all, there seems to be a very different audience playing it, and the feel of the chat channels and city general channels is very different. If you thought the trade channel was chaotic in Ironforge, just wait until you get into the starter area of Guild Wars -- because the game has no AH, not only is there more crazy business to be done, but people playing the game for free seem to have even less of a conviction towards making it a good experience for everyone else. WoW's community is a little crazy, but at least there are a few good people out there who'll group up or give you a port to Shattrath. In Guild Wars (in my experience), maybe it's a combination of the instanced world and the free-to-play quality, but it's very much an every-man-for-himself game unless you're playing with friends you know.
Not that GW is a bad game -- I enjoyed it, and still jump in occasionally to do some questing. But it's very different from World of Warcraft and many other MMOs.