Although many people still consider it as such, Guild Wars is not a MMORPG as it is currently known. ArenaNet itself uses the term Competitive Online Role-Playing game. Although they're not really in the same genre, I think the strengths and weaknesses of Guild Wars can be explained best when compared to World of Warcraft.
First of all there is a level cap of 20. This may same weird if you're used to WoW, but this is because of the way your skillbar works. You must come up with a build of only 8 skills that complement eachother, using the skills available for your primary and secondary profession (your primary profession allows one of its elite skills). So you have to think strategically about your skillbar, and you don't have to micromanage 100 different skills. All the skills are carefully balanced and diverse, resulting in many interesting combinations.
Secondly there is the absence of many fetch quests, and being forced to grind. The PvE content is largely built around 4 decent storylines (the "Factions" story is mediocre) consisting of diverse missions, with only small amounts of grinding involved. "Factions" for instance requires you to grind for reputation within a faction, but even then you can do a range of individual quests that reward you with faction points, which are generally more diverse than the average fetch quest.
The two features above allow the game to put a bit more emphasis on actual player skill instead of time spent playing the game. Other nice features are a strong PvP system, high production values (talented art department and music by Jeremy Soule), and of course the lack of subscription fees.
There are also disadvantages to the game. The biggest disadvantage is the instanced world. To me, WoW is so attractive because it has a huge persistent world. While the entire Guild Wars world is also quite big, you can only meet other players (outside of your questing party) within towns and outposts. Once you leave their gates, you're playing in an instance of a certain area. This makes the gaming experience a lot less immersive.
Other disadvantages are the lack of different player races (you're forced to play as a human) and the complete lack of a trading system. I still don't really know why but if you want to trade with other players, you just use the local chat. This is incredibly annoying and time-consuming. Something like the Auction House from WoW would be wonderful but there' s just nothing there.
To sum it all up:
+ Quality PvE and PvP experience
+ A bit more emphasis on skill instead of time spent
+ Well-balanced professions and skills
+ High production values (Art / Music)
+ No monthly fees
- Instanced world
- Only one player race (human)
- Non-existent trading system
I could have mentioned more aspects like guilds, but hopefully this review cleared things up a bit about the important differences between Guild Wars and a real MMORPG like World of Warcraft. All things considered I still prefer Guild Wars, but make up your own mind about the pros and cons when choosing an online RPG.