Guild Wars has brought some truly revolutional ideas to the MMO space. Sadly, not many of them are being adopted in upcoming MMO's, but, if I was making an MMO I would certainly learn from all of the following (in rough order of importance):
1) PvP characters. In Guild Wars, you can create a PvE character and play normally, but it takes a while to get "decked out" to be competitive in PvP (as with any MMO). So, there's also the option of creating a "PvP Only" character, who essentially has access to all skills and items, but who can't engage in PvE content.
Basically, it lets you be competitive with everyone else in PvP without investing all the time do so. If you feel like dabbling in a particular class, trying out a build here and there, you can do it in 2 minutes, as opposed to the months of playtime it would take in most other games.
2) Grind for looks, not power. Guild Wars makes every effort not to be a massive grindfest timesink. They do this by:
i) Making it take very little time to hit max level. You might think it would be boring because there's nowhere to develop your character once hitting the level cap. Not true! You'll only have just started to unlock all the skills available to your class (and all the useful secondary class skills), and if you want to be able to re-spec your character to play a variety of builds, it will take a good amount of time to do so.
Plus, there's all the end game content, levelling up titles (read, achievements), etc. Most people who PvE for a little while in Guild Wars will have several "maxed out" characters to experience end game content with. That's right, you can actually play an MMO and get to try out all the classes at max level! Contrast this to WoW where it takes most people months and months to get to the level cap for just one character.
ii) Weapons/armor of max power are cheap to buy. But the downside: they don't look very "leet". If you want the cool stuff, then you put in the grinding. I you just wanna get kitted out with max gear and not fuss around for months trying to get uber lewt so you can join some hardcore raiding guild, you can do so and you'll be just as competitive in every way (just a bit less attractive :p)
3) Easy respec. In Guild Wars, you can respec while in any town. Completely. Costs nothing. 'Nuff said.
4) The combat system. Most MMO's have healers to mop up damage, ie "make red bars go up". This is traditional, and boring.
Guild Wars introduced the concept of "Protection" skills, which are cast on allies before damage is inflicted on them, with the goal of avoiding said damage. These types of skills are much more efficient than healing skills, and are really good for the game because they add an extra layer of depth and skill in terms of reading the play of the other team.
5) Observer mode. Every single top 100 team PvP match and battles in the "Hall of Heroes" are replayed on a kind of in-game TV called observer mode. Pushing "B" while in any town brings up a list of all the games you can watch. You can see what all the skills both teams use are, when they use them, what their movements are around the map, etc.
This simple feature has had a ridiculously powerful effect on the skill level of players playing the game. If someone comes up with a new team build that dominates, you can be sure they'll get to observer mode, and the build will spread like wildfire. This helps the game to be less about who's teched out with some secret build that the other team wasn't aware of, and more about who's got the best tactics to win.
6) Map travel. Oh, how I love thee. You open your map, click on the town you want to go to, and you're there. Don't get me wrong, I know some people love cruising around on their mounts in WoW, taking in the sights. I just get really bored of this sort of thing, really fast. Don't see much reason why there can't be both map travel AND nice scenery/mounts, and people can get to wherever they want to go in whichever way they please.
The remaining points are more minor, but only because the above points are so major:
7) The skill system - instead of pumping up individual skills, your points pump up the entire tree, which both seems to make more sense "realistically" speaking, and makes it easier to try out/incorporate new skills in your chosen skill tree as you come across them.
8) Short cooldowns on skills, with appropriate balance. Games like WoW are full of skills with massively broken effects, that are "balanced" by putting them on very long cooldown timers. I don't like this style of play, because you always feel like you're waiting on your favourite skills. In Guild Wars nothing has a cooldown longer than 60 seconds, and the vast majority of skills recharge in 20 seconds or less. Using skills is fun, remember?
9) No monthly fee. I've been playing Guild Wars for ~3 years now, and buying the game plus all the expansions etc cost me ~ $250 (Australian) total. If I was on a WoW-style subscription, I'd be looking at $540 from monthly fees alone, let alone the cost of the game boxes! The cost factor aside, it's also nice to know I can take a week or 2 off without feeling like I'm wasting my money.
Think of anything else that this game, or any other, has added that you think should become the 'gold standard' in fresh MMO's on the market? Think my ideas are crazy B.S. and want to rip me to shreds for mildly bashing WoW? Share your thoughts.