Explaining Guild Wars
Guild Wars is almost an MMORPG. It's not really an MMORPG, although it looks a lot like it. The way that the game works is that there are two types of areas: "towns" and "areas". All the areas are instanced, so you never see people outside your party in them. You can only meet new people in towns.
Now, stripping away all the flavour for a minute, what does this mean? It means that we have a graphical veneer over lobbies and a straightforward multiplayer online game with a low number of players - max 8 and lower than that for lower-level areas.
As you may know, bandwidth costs vary with N^2 where N is the number of players. So this means that NCSoft's bandwidth costs aren't very high - the only places where N is high are the towns and you can't do an awful lot there. Hence, they can get away with not having a subscription costs and just eat the bandwidth and server costs as part of cost of goods, just like Diablo did. The major difference is that most of Diablo's players were singleplayer whereas all of GW's players are multiplayer.
The game itself is a mix of two types which are almost completely separated. There is a "PvE" (player vs. enemy, or AI) section which plays mostly like a normal MMORPG and a PvP section with several different sub-game-types. The PvE is competent and the PvP is exceptional, simply because that was the focus of the game from a design perspective.
The key game mechanic is skill choice. A given character has a primary and secondary profession, each chosen from a pool of 6. The available professions are Elementalist, Mesmer, Monk, Necromancer, Ranger and Warrior. The primary profession gives you access to a few more skills and skill-enhancing equipment that works for the skills of the primary profession only. Each profession has approx. 75 skills so once you unlock all of them you can choose from around 140-ish. Except that you can only use 8 at a time - you can reset them whenever you go to a town. In fact, you can reassign skill points in towns too so you can nearly completely "re-roll" your character whenever you want. The same mechanic applies in PvE and PvP. This pre-choice mechanic gives the game a strong flavour reminiscent of Magic: the Gathering - given character "builds" will be more or less effective depending on what the opposition is doing.
Now, technically the game isn't strictly multiplayer. GW offers a powerful feature that the real MMOs don't have - henchmen. Henchmen are serviceable AI companions that will come along with no (gratis, even) to fill in spots in a missing party. This means that you can play nearly all the PvE content on your own if you wish. This is ideal for a casual player, since you can get halfway through a mission and simply find a safe spot and leave the character sitting there with the eternally patient henchmen.
In general, GW is much more friendly to casual players than traditional MMO designs. Travelling between towns? Sure, it's fun to ride that horse/griffin/whateveritis for the first time - but by the 3rd you've seen it. In GW, you can just magically teleport between towns (no explanation given for this in flavour). This pretty much sums up ArenaNet's approach to the game design - if it improves gameplay, they did it, even if it doesn't fit with the flavour of the game. If I only have 1/2 an hour to play, I can do it and (this is the key part) still feel like I accomplished something. Death penalties are all reset if you go to a town area so it's very low-risk.
Ok, now onto the PvP. There are 3 base kinds of PvP: Arena, Guild vs Guild and Tombs of the Primeval Kings. Arena is a straight-up 4v4 match with two variants. "Competitive Arena" has randomly chosen teams where you keep the same team if you win. "Team Arena" is the same thing except the teams are pre-chosen by the players. Guild vs Guild is a 8v8 match where the object is to kill the opposing Guild Lord (a NPC). The maps for Guild vs Guild are the lower ranked guild's Hall (fixed for the guild, picked from 6 choices). The maps feature a bit of Capture the Flag, a sprinkling of NPCs (archers defending your hall), basic fixed siege weaponry and so on. Tombs of the Primeval Kings is a 8v8 tournament format where the object is to get your Ghostly Hero (a NPC) to the final stage and have him hold the altar in the final map for a period of time. In the tournament you fight sometimes 1 team at a time, sometimes more (always more at the final stage).
Obviously there's a lot more details than this but that's the basic summary of the game. Hope that was useful if you didn't already know it. I'll probably be posting more actual analysis at a later point.