In many ways, Guild Wars is more like DAoC than WoW. I've played both of those games heavily for a 2 year period, so obviously I enjoyed them very much.
Guild Wars is important for its differences.
-Character levels exist with a level cap of 20 but leveling isn't a focus of the game at all. Gathering skills and trying different builds for the various encounters is the on-going challenge.
-Professions, also known as character classes, are unique in and of themselves but every player chooses a Primary and then a secondary profession. This mixes up the deck a lot if you are used to the usual pigeon-holes that character classes fell into in other games. The "Holy Trinity" of tank, heals and DPS is still partially there but...it may often be hard to recognize. But, if you play a Monk, people will still expect you to pull their bacon out of the fire more often than not.
-And, DPS...well, damage per second isn't something that belongs in Guild Wars jargon because it isn't really the governing factor. In DAoC and WoW, the talk of DPS entered into nearly every discussion, but not so in Guild Wars. I think this is mainly because there aren't really zergs in Guild Wars, but, I will defer the the lack of DPS-centricity to someone else. I can only tell you that it isn't a governing concept.
-Each chapter/campaign has a different feel and pacing but each campaign is connected to the other campaigns (if you choose) to make an even larger game world. The game is centered around the concept of "unlocking' areas (as well as skills). As you progress through a given chapter/campaign you will find your self opening up new areas. Of the three chapters, Prophecies has the more open design that allows you to "run" or bypass mission paths and quest hubs. Factions and Nightfall take a more restricted approach. Needless to say, after you have successfully progressed through an area you can return there at anytime.
-Instancing: when you leave a town or outpost you and anyone grouped with you will be in your own private explorable area. This may seem weird at first because you won't be running into dozens of other players or trying to compete for a quest mob. On the other side of the coin, while there is no one there to hinder you and your party, there is also no one there to save you if you get into trouble. The Death Penalty takes effect upon death and reduces all your stats by 15% for each death up to the max penalty of 60% reduction. The good news is that this DP goes away upon entering a town or outpost.
-PvP, while I haven't PvP'ed nearly as much in Guild Wars, I can tell you that the dynamic is completely different. Battles generally last longer and AoE doesn't play quite the same role and mass crowd control is simply non-existent. I am stating these as positives, not negatives, lol.
-Gear and Weapons are balanced, it's more about unique/rare skins than stats although there are a variety of stats to compliment various and particular builds. But, no Armor or weapons to make you god-like.
-Skills, however, can be collected and some are categorized as "elite" and can be captured from defeated Bosses. Elite skills are generally powerful and useful and play a large part in successful builds often as "cornerstone skills". But, in the interest of balance: only one elite can be on your skill bar at one time.
-The game seems simple to the new player and then starts gathering subtleties as they progress further. Various skills and builds make up a lot of the variety but when combined with player teams and the many types of enemies you will encounter and the mission challenges, you end up with a very varied experience.
-Guild Wars is a game you can set aside and come back later with out feeling you have "fallen behind". In DAoC and WoW if you left for a 3 week vacation, you had a lot of catching up to do when you got back to get back on the same level as your guildees, etc. (Master Levels, Trials, Raids, Loot Drops, DKP, etc.) Missions are the closest things that Guild Wars has to raids, but they don't last 5 hours or require 40 of your "closest" friends to accomplish. There are high ends areas that can take quite a lot of time to clear but the rewards are greater as well.
OK I've rambled enough, feel free to ask questions or browse the GW-Wiki:
http://gw.gamewikis.org/wiki/Main_Page