So, what are classes like? Do they adhere to the DPS/tank/support roles that have grown obligatory to MMO's?
Yes, no, somewhat, and it really depends on what you mean.
See, the thing is, since every player gets a primary class and a secondary class, it's possible to have every skill in the game known, but you're limited only to those skills available to you that you get from your specific classes. As a result, most classes have a fairly broad mix of DPS, tank, and support skills, so sometimes, with the right skill combos, it's possible to have "tank" or DPS classes that mostly play a support role, and also DPS and support characters that tank.
However, generally certain classes are better at certain things than others. A Mesmer primary is rarely suited to tanking, for example, while a Warrior primary generally has good armor and has a lot of skills that make them better at tanking, but are also generally poor spellcasters, and simply cannot be as good of a combat interrupter as a Mesmer is, (debuffs and skill interruption are the Mesmer's specialty, and among PvPers, good Mesmers are considered to be quite dangerous opponents). Monks have few native skills that let them DPS or tank, though with the right secondaries, they can do at least well enough to solo, but they are excellent healer.
That said, it's often possible to be rather effective in a non-traditional role. One build, the 55/105 Monk aka the I-Monk, or Invulnerable Monk, is a Monk/Warrior or Monk/Necromancer who uses Monk skills and their secondary to become essentially invulnerable to damage, in a certain limited fashion. However, despite the name, it essentially has no presence in PvP, as it relies on using easy to dispel enchantments and has low damage, although it can often be an excellent farming build. I personally played a Ranger/Monk as a solo healer through what was at least at the time, widely considered the most difficult mission in the game, with perhaps the worst PuG I've ever seen, and that with half the group size traditionally required and without a second healer, as was considered necessary according to the prevailing wisdom. (I honestly don't suggest it. I was just that tired of being held up waiting for a healer and wanted to move forward in the storyline.) Plus, since all classes are meant to have something to appeal to players of all types, there's often skills in your primary and secondary which can be used in a synergistic fashion to create sometimes stupidly effective techniques. The 55 Monk is one, though there is also the Ranger/Necromancer "touchy Ranger" who uses necromancer abilities as his main attacks, and others. The thing is though, most of these combos are historically rather ineffective at major group-level PvP, which is what most things are balanced around, and when they do become unbalancing, ArenaNet is generally quick to patch the game.
There are two things Guild Wars does to ensure players can't totally usurp traditional class roles: first, they limit armor availability and most armor enhancements to a player's primary class, and secondly, they have certain attributes which give inherent bonuses that are only available to a player with that primary class. This also means that certain skills will tend to be used exclusively by players with that primary, because they are linked to that primary-only attribute. They may not be (Nearly all skills, for example, grow better with an attribute, but some are good enough that players may still use them even if they have a 0 in that attribute, or are designed in such a way that players knowing the limitations of that particular skill can build around it in a way that lets them be effective with that 0.)