Every class gets spells or combat arts (automatically when you level, you don't have to buy them). These are things like attacks, heals, buffs, etc. Many classes would be in major trouble if they just hit auto attack and walked away; you have to use your extra attacks if you want to get anywhere. When I solo my swashbuckler and my fury, I'm constantly clicking icons until the mob is dead. I have a baby summoner who can actually send her pet out to kill weak things by itself, but it's not worth it. I kill stronger enemies and help my pet out by casting damage over time spells on its target.
Classes are reasonably balanced, and there are some major combat changes coming that will balance them even more. There aren't really any unwanted classes, and the new changes will help some classes become even more useful (like enchanters).
No cookie-cutter parties. It is true that in a full group, a tank (fighter) is very nice, and that as you level, you may need one or even two healers. However, I've been in plenty of small groups that were missing one or both of those things, and they worked just fine.