Guild Wars has changed MMORPG's world in a very good way.
For starters, GW has a great storyline, something that most free MMORPG's don't have. The plot is epic and full of twists, keeping you interested in what is going to happen / what happened. The missions are epic and challenging, with good cutscenes and a side quest (bonus), for players that like to things the "hard way". The quests introduce interesting side stories form Tyria's citizens and are fun to do.
The landscape of Tyria is astonishing, from the beautiful Ascalon from the Pre-Searing to the snowy peaks of Shiverpeak Mountains and to the fiery Ring of Fire Islands. The game's graphic design is very good for an online RPG.
But the real change GW has done to this type of game is the actual gameplay: to be a good player, you don't need to spend hours training (since the level cap is up to 20); you need to have a good tactic. your skill bar can have only 8 skills, and only one can be an Elite Skill (the most useful skills of the game that are obtained by killing a boss who has the skill and capturing it with a special signet), so you'll have to combine each skill in order to make a tactic against some type of mobs (for example, a skill bar with lots of Area of Effect damage is useful in areas with lots of enemies). Although there are only 6 professions, you can choose a secondary one, giving you the chance of creating a vast amount of builds (a build is a full skill bar). Despite not being able to go beyond level 20, players are distinguished by their titles: there is an exploration title, a title for who beat the game with all bonus from the missions done (and the missions too, of course), etc. But don't be eluded by the many tactics that you may use; mobs in this game have a great A.I., and they will always try to find the weak point of your strategy: They will attack your healer, if you use enchantments they will try to strip you from them...
Other good aspect of the game is the weaponry and armour sets GW provides. Weapons can be customized with parts that grant bonuses (attribute and armour boost, more energy...) and can be dyed. Some unique weapons are dropped by bosses, although they can't be customized nor dyed. The armour sets are really good looking and can be customized and dyed. You may want to combine pieces of one set with pieces of another, giving you ann unique look amongst other players. The armour pieces can be crafted (by the crafter) or obtained by a monster's trophy collector. The elite armour sets can only be crafted, but they cost a lot of materials and money.
By the end of the game, you won't leave GW so soon: you have new hardcore quests, the Hard Mode( where enemies are way stronger and smarter than in Normal Mode) and the Battle Islands, where PvP happens: GvG, team battles, etc.
However, GW is not a perfect game. The skills' special effects aren't very good, so don't expect big explosions or great swordsmanship moves when you are fighting.
The henchmen (NPC's that you can invite to your party in order to fight alongside you) you bring with you in your journey aren't so smart, and they can ruin your strategy.
But the worst aspect of GW is the lack of communication between players, because you can only meet other players in towns, outposts and mission outposts, and not in explorable areas, giving you an hard time to find someone who wants to complete the same quest as you do or to farm the same boss as you do. Unless you have a guild, you may have to spend most of your playing time doing quests of trying to get materials all by yourself.
In conclusion, GW has changed the world of MMORPG's and deserves nothing less than 9.5.