If you're only at L20...you haven't explored the class yet unless you've done some serious sPvP!.
You haven't tried the elite skills, they're fun to play with, and you don't even have Master level traits yet! Rangers get multiple stability sources, letting them shrug off some really annoying boss fights and encounters.
Traits are one of the most fun things to play around with IMO. You can drastically change the way you play the game with a couple of clicks. Well...maybe not for warriors. One of the reasons I never played warriors past beta is because I never really liked the way traits worked well together (a shouts build is a notable exception, but I never delved too deeply).
Mesmers have the best trait selections in the game for tweaking, messing around and synergizing with themselves. See these
theorycrafting threads, for example.
The leveling system itself is made so it gives you something cool every 5-10 levels, be it a new skill slot, a major/minor trait, access to a new dungeon, weapon/armor tiers etc...
Of course, you can always get some quick levels through crafting to see a real change in your character.