I've been a player of Guild Wars for a little over 3 years now. Just thought I would give you my thoughts on it.
As has been mentioned here, it's not an MMO in the truest sense by any means but there is still something within that can make a person spend the same amount of time on it as they would with WoW.
Now, in spite of this, it does have it's fair share of problems. I'll divide up into PvE and PvP what I think the main ones are.
PvE:
The lvl 20 cap really isn't a problem imo. There's still some challenging elements that you can tackle much like WoW raids. The sense of accomplishment isn't quite the same though.
Once you have completed a campaign, or even all of them + Eye of the North, there's really not much to do unless you want to grind. Initially ArenaNet (The developers) said they want it to be as grindless as possible. Sadly, they have failed and the addition of titles has emphasised that, although they are completely optional, they do add longevity where there essentially is none so I guess there is some balance. Where Prophecies is concerned at least, there's less grinding. Factions and Nightfall on the other hand have a couple of mandatory grindy sections that are part of the storyline.
Probably the biggest problem of all though is with Prophecies itself. It's been out for a real long time now and at present, there's just not many people playing it from the start anymore (although, hopefully the ultimate collection or whatever it's called will change that), so getting a party to play through the game will be tricky and stunt your progress. If I were starting now I would probably get pissed off that certain sections will be very difficult to get through.
One way to get around that would be to get into a guild. This in itself becomes a problem. In spite of the game having a good community, there's a lot of bad PvE guilds that are very elitist and not very willing to help. I'm not saying they're all like that though, I just can't say that most aren't like it either.
Other than that, the campaigns on the whole are enjoyable enough. There's a nicely crafted lore behind the game too and it's established itself well.
PvP:
OK, I confess I'm not a big PvP player. Never have been and never will probably. I do do a little occasionally but I do it for fun rather than competition.
The core of the PvP element is something called Heroes' Ascent. It's just a perpetual tournament winner stays on kind of thing. It's fun but it's really hard to get into a team. The vast majority demand players with a good rank and of course to get rank you have to be able to get into a team in the first place. It's frustrating.
Also, generally in spite of it being more about skill than anything, it's very common to see teams with the same builds time and time again. This is the case in pretty much every element of PvP. Random Arenas, Heroes Ascent, Alliance Battles, Hero Battles and Team Arenas. The same people that copy their builds from websites that result in being a dime a dozen everywhere you look also tend to be the same people that sneer at you for having the balls to even try to get into PvP. Basically, it's just the same as every competitive online game except your team gives you stick as well as your opponents.
If you just keep on with it and try and do things for your own pleasure and do things your own way though, it is fun.
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This probably sounds like I've bashed it a fair bit but I think it's only fair to give a good warning first
As for the missing link of everything, Eye of the North, the only real Guild Wars expansion, there isn't a hell of a lot to it, but it's there mainly if you ever have any intention of playing Guild Wars 2. That's pretty much all it's there for. It acts as a bridge to the sequel. It's alright. Nothing special but it's definitely one for the fans. It's hard to say much about it. It doesn't really add anything to any of the campaigns as such.
Other than that, I do love the game personally and have met some great people on there in the last few years. My best tip I can offer is if you are getting it, get yourself into a helpful guild. I'm sure Vampire Hunter D will help you out there
Enoy Guild Wars!