Guild Wars has many strong innovations, but ultimately many flaws as well.
In many ways, Guild Wars is a strange MMORPG. It does away with grinding (mostly), it focuses on skill, it does not have monthly fees. You can beat the entire game with AI-controlled bots, you can map between ouposts in the blink of an eye, you never meet players not in your party while adventuring. All this gives Guild Wars a feel unlike any other.
In Guild Wars, you play a hero whose background depends on which continent you begin on (i.e. if your character is from Prophecies, Factions or Nightfall). All three games have their own problems and backstory, but chronologically they occur at about the same time, and a character can travel across campaigns with no problems. The skills your character learns in the beginning vary from campaign to campaign, and some primary professions have to be from a particular continent, but ultimately everyone is equal. A Mesmer started in Prophecies will end up just as powerful as a Mesmer started in Factions, with the difference of at most the appearance.
For players of other MMORPGs, this character equality is so thorough it might come as a shock. When I say the Mesmers end up just as powerful as each other, I don't mean that they are just as powerful only for serious players who spend hours farming for a specific item. No matter how much one might farm, an Elementalist in Guild Wars will never find armour with base armour rating above 60. Sure, there exist so-called "prestige armour" that cost much more to craft than standard armour, but functionally they are the same. There is nothing - no extra health, no extra mana, no extra defense - that you can find on prestige armour that you can't get on standard armour. This equality extends not just to armour. Weapons, too, are largely created equal. There are some exceptions here and there, but the differences are very minor. The result is that casual players will end up just as powerful as hardcore grinders, with a maximum difference of cosmetics.
In some MMORPGs, a different bar is imposed by how long one is willing to spend leveling up, but leveling up in Guild Wars is extremely easy. A character can reach a maximum of level 20, with 200 attribute points to spend. Hitting this cap is easily achievable. Simply playing through the story of your chosen campaign will get you to level 20 and max out your attributes. This doesn't create problems with Factions characters joining the Nightfall campaign because of the speed at which you gain levels. Especially with Factions and Nightfall, you can hit the maximum level even before leaving the tutorial areas. In this equality, Guild Wars scores well. Those players interested can still farm hard and equip the rarest of items, but casual players aren't left behind.
All these innovations also mean that PvP in Guild Wars is remarkably level. Because everyone will be at the same level and use equally strong items, it is player skill and build choices that determine who wins. You do not even have to go through the motions of creating characters and maxing your attributes to get into PvP. In Guild Wars, you can create PvP characters that start at level 20 and have the best armour and weapons. This makes PvP easy to get into.
Another strong innovation in Guild Wars is the flexibility. In manny MMORPGs, a character's "build" is a very delicate thing. For the maximum power, you need to learn specific skills at specific levels and find specific weapons. Make a mistake and the scar will stay on your character forever. This delicacy simply does not exist in Guild Wars. In Guild Wars, you can equip a maximum of 8 skills, the power of which are determined by your rank in its associated attribute. For example, the spell Fireball does damage based on your rank in Fire Magic. One might think that choosing whether to increase your rank in Fire Magic or whether to equip Fireball is a "maximum power" decision. Not so! You can change the skills you have equipped freely every time you are in town. You can even change your attributes with a simple click of a button. An Elementalist that throws Fireballs today might be throwing Lightning Bolts tomorrow. This flexibility leads to stress-free play. Not only are you free to try out all possible options, you do not need to restart their characters to correct a build decision made long ago.
There are 10 professions in Guild Wars, and the interaction between them runs deep. Each profession has a primary attribute (whose rank only that profession can increase), together with 3-4 other attributes. This is not a game balance problem - it is difficult to use more than 4 attributes at the same time. You choose a a primary profession when you create a character, such as Necromancer. Then your character can learn a secondary profession. This lets you equip skills from that profession, as well as increase the relevant attributes. Essentially, this means your Necromancer can throw Fireballs just like an Elementalist can. This doesn't make you just as effective - only an Elementalist primary can use runes that increase Fire Magic beyond the normal cap of 12 - but it is still an option. Because of the sheer number of skills and professions in the game, this allows for almost limitless customization.
And that's only for a single character. You can form parties of up to 12 players in Guild Wars, although most areas have a limit of 8. Where you can't find other players, it's possible to plug in the gap with heroes. These are AI-controlled bots that use the exact build you wish. Although not as effective as a human player, they are reasonably competent, and everything in the game can be cleared using just heroes. Nonetheless, because bad human players can actually be worse than a hero, and because other players may just quit if a mission isn't going well, this does encourage solo play.
Content-wise, Guild Wars is huge. There are hundreds of areas to explore and many dungeons to clear. The missions are numerous and generally not repetitive. Difficulty comes in two levels. You can enter a mission or area in Normal Mode, where mobs are lower level and have weaker AI, or you can try in Hard Mode. In Hard Mode it's very possible for mobs to be level 28 or even higher, or 8 full levels above the player cap. Nonetheless, the game is quite easy, and as long as reasonable builds are used, casual players can clear most areas even in Hard Mode.
Still, Guild Wars has several serious problems. One glaring fault is the storyline. Depending on which campaign you start with, you might be trying to help your people escape an invading force, or you might be investigating a strange plague, or you might be dealing with a demon invasion. But ultimately, all these storylines are shallow and occasionally nonsensical. One of the worst culprits is the so-called "War in Kryta", a part of Prophecies. In this story arc you're part of a rebel gang called the Shining Blade who are ostensibly fighting a guerrilla war to take down the tyrannical rulers of Kryta, the White Mantle. Incredibly, one of the first things that happens is that the Shining Blade seizes the kingdom's capital and most populous city. What the heck? Is the Shining Blade a guerrilla force or are the White Mantle just morons? Even more incredibly, when the White Mantle comes to take the city back, your commander orders the front gates open and the battle fought within the city itself! Why someone would willingly forfeit the defenses afforded by fortifications is beyond me.
Furthermore, the role-playing element in the game is severely lacking. There are no story-changing choices to be made. You cannot for example choose to side with the White Mantle instead of the Shining Blade, or choose between different endings. There are a few dialogue options to choose between, but they do not alter gameplay. The game forces you to be what the the developers want you to be, which doesn't leave much room for roleplay.
Another major fault with the game is game balance. The game sells Elementalists as the premier damage dealer and Monks as the default healing profession, except that this turns out to be totally inaccurate in practice. Elementalists not only turn out to be pretty bad damage dealers (with other professions capable of dealing over twice their damage), they are also the best healing profession in the game. This is fairly common knowledge among the community, but not only does ArenaNet not take steps to fix this imbalance, they actually seem unaware of it. There currently also exist Assassin builds that make them (essentially) permanently invulnerable. This power allows a single Assassin to clear some areas faster than 8 players not using that invincibuild, yet still the developers refuse to hit the skill that makes this possible.
You might think that the situation is better in PvP, but it is not much so. Over the years many previously-viable skills have been weakened to the point they aren't usable anymore. The result is that what used to be magnificent build diversity is now reduced to a trickle. Though the game has hundreds of skills, there really are only several worth considering. It's not a matter of choice here: you HAVE to use those skills to stand a chance of winning. Some balance decisions were so strange they have even entered popular use in the game. The Monk skill Smiter's Boon is an example. In one balance update this skill was hit extremely hard: its energy cost was quintupled, its recharge was tripled, its duration was cut by a factor of six. This gave rise to Smiter's Boon (verb): to Smiter's Boon a skill is to weaken it so badly that it is essentially removed from the game. This balance decision wasn't an accident either - the developers said they knew they were essentially killing the skill entirely. So much for the ideal where each skill is a viable option.
Yet another alarming change is the recent introduction of mercenaries to the cash shop. These are essentially heroes, but unlike normal heroes, they have the same appearance as your other characters. This sounds like a cosmetic change, but there is a hidden in-game advantage. A player can normally use only 2 hero Mesmers, because there are only two Mesmer heroes available in the game. However, with mercenaries, you can now fill your party with 5 or more Mesmers. ArenaNet has acknowledged that this provides an in-game advantage, but argue that it is a case of more options, not actual power. Sadly, once again the developers are out of touch with reality. Mesmer heroes are strong, and being able to mass Mesmers provides a real advantage.
That said, Guild Wars is still a fine game. Its skill-based, customizable ideals are some I wish other games will take after, and its content-loaded worlds are places where I've spent hours wandering without regret. Best of all, it does have no monthly fee and little content available only to those who pay real money - one of the only decent MMORPGs to be so.
Pros: skill-focused; highly customizable characters makes for stress-free gameplay; lots of content; no monthly fee; easy-to-get-into PvP
Cons: lack of role-playing; weak storyline; bad balance decisions; recent trend of updates has been mildly alarming