Guild Wars features:
6 different professions with the ability to have two professions at once, a primary and a secondary.
Action packed fun PvP with teams and many different arenas and locations.
Tournaments and ladders, bring your Guild to the top!
Play alone or with friends, your group has its own instance of every adventure zone.
Tons of quests for solo or groups, coop missions and more.
Have your own Guild Hall and use it as a base to plan your domination of the Guild Wars.
No monthly fee! Play as long as you wish in a huge online game, pay only for the expansions packs that you want to get.

Originally Published by THX

Professions

6 Professions [ Elementalist, Mesmer, Monk, Necromancer, Ranger, Warrior ]
Can "Dual Class", a total of 30 different profession combinations
Roughly 75 skills per profession
Access to all primary profession skills, but do not have access to all secondary profession skills. The secondary profession lacks the "primary attribute", thus a whole skill set is unavailable.
Armor is profession specific
All professions can use all weapons. However, weapons often have attribute requirements, thus if the req. are not met then the character will not be as proficient with it as a character that does meet the requirements [ie. damage output will be lower than what the weapon's potential output is of]
Only rangers have pets
Appearances are profession specific and are quite different from each other
Skills

Attributes increase the effectiveness of linked skills
Not all skills have a linked attribute, but most do
Elite Skills are not superior skills, however you may only bring a single elite skill into battle. Such skills would be too powerful if combined with other elite skills, thus the limited use.
There is no "ultimate" skill, each is very balanced
A successful player correctly uses a wide variety of skills and adapts to the situation, there is no cookie-cutter "ultimate" build.
Skills may be acquired from Skill Trainers, Collectors and/or captured from Boss Creatures.
Skills may only be captured by using a Signet of Capture
Some skills have energy upkeep, a unit of upkeep removes one energy renegeration arrow. If the upkeep exceeds the number of renegeration arrows, the character's energy begins to degenerate
Skills with energy upkeep will remain active as long as they receive the required energy. They can be disabled manually.
Many skills do not inflict instant damage and instead cause Damage Over Time.
Only 8 skills may be taken into battle [including 1 elite skill]
Adrenaline is acquired by successfully striking a foe. Only Warriors have adrenaline based skills
Is Guild Wars similar to...?

Diablo? Only in style, the game is immensely more complex than Diablo. Specifically, anyone could play Diablo effectively, while Guild Wars requires excellent team work and tactical thinking.
Counter Strike? Not exactly, Guild Wars PVP is fast paced and requires a degree of skill akin to FPS multiplayer. However, the application is drastically different. [remember, it is an RPG and not an FPS]. Nevertheless, GW PVP is likely the most similar to FPS multiplayer games than any RPG on the market [due to the emphasis on skill rather than equipment and statistics].
World of Warcraft? Somewhat, however there are many drastic differences beyond the "large world" atmosphere. ie. Guild Wars features Excellent PVP, little of agrind, less emphasis on levels, fast paced combat, less time required to be effective etc. On the flipside, Guild Wars does not have races
Is Guild Wars a ...?

Hack and Slash Game? Only in its more simplest format, the true gameplay is derived from the use of skills and not hack and slash.
Traditional MMO? No, it uses many MMO elements; however much of the grouping is done in instances [non-persistent]. However, everything is server based.
Battle.Net like game? No, the world is run off the ArenaNet servers just like any other MMO.
PVP only game? No, it has a very large PVE component.
PVE only game? No, it has a very large PVP component. Essentially, both the PVP and PVE aspects have been fully developed.
Pay to Play game? No, it does NOT have a monthly fee.
Does Guild Wars have...? [does]

Pets? Yes, the ranger may acquire a pet.
Player-Versus-Player? Yes, the game has an extensive pvp component. Many claim it is the best PvP around.
A death penalty? Within a mission, the character will suffer a 5% death penalty per death [removes 5% health and Energy]. It vanishes if the character exits the mission (PVE or PVP)
Customizable Capes? Yes, Guilds and Guild Members may customize and acquire capes
Guild Area? Yes, Guild's may acquire a Guild Hall
Guid vs. Guild battles? Yes, Guilds may attack another Guild's guild hall.
Quests? Yes, many quests are available. Including the conventional retriever quest format.
Bosses? Yes, most maps have a handful of bosses. Such monsters are larger and more powerful than their more common "normal" siblings. They may be easily indentified due to their glowing aura.
Tournaments? Yes, groups/guilds compete to become the single most poweful guild in-game. In such instances when a group reaches the top of the ladder, it's emblem is plastered all over the world of Tiria in form of statues and flags.
A lore? Yes, Guild Wars features an extensive lore created by the writer of Baldur's Gates lore and various Dungeon and Dragon novels.
Trade Market? Many players actively trade their acquired goods, there is a lively barter market.
Different item types [labels]? Yes, there are sub-par, normal, magical and rare items.
Effective anti-cheating measures? Yes, Guild Wars is almost entirely server based thus it is much easier for the developers to prevent cheating.
In-Game Events? Yes, every now and then Guild Wars features in-game events. ie. a quest or extension of the story.
Does Guild Wars have...? [does not]

A monthly fee? No.
Do I need to buy expansions? No.
Will there be expansions? Yes.
Mounts? No.
Talents? No.
Crafting skills? No. [although, there are NPC crafters]
Dual Wielding? No.
Aunction House? No.
More than a level 20 cap? No.
A sleep option? No.
An Inn? No.
Spectator Mode? Not yet, however it is in planning.
A grind? A bit, but hardly any compared to conventional MMOs.
Skill Levels? No, effectiveness of skills is determined by the level of the linked attribute [attributes often affect a great many skills, not a single one]
Player-Owned Housing? No.
Duals? No.
Player-Killing [ie. outside of agreed-to PVP]? No.
In-Game Voice Communication? No, however many use 3rd party software, "Teamspeak" or "Ventrillo".
In-Game Recording? No, however many use "fraps" to record videos.
"Flying"? No.
Giant Robots? No.
Firearms? No.
Potions? No.