Guild Wars had far better balance than WoW ever has had. ANet had constant balance fixes. At a certain point, they went overboard with balancing, but their ability and willingness to make balance tweaks is impressive. They split PVE and PVP skills at one point to further help with this. It's only when they got to the point where they decided they wanted to suffocate build wars that things fell apart.
A little story: One night I was running Halls of Heroes in Guild Wars (A Tournament PVP mode). We were running an e-drain, KD warrior build, but heard that there was some new build going nuts. Someone had found out a way to use Warrior/Elementalists to get an elite skill that caused interrupt on KD to trigger over and over again by wielding lighting axe hafts, but without the energy draining drawback to the skill. This was pretty cheese and was facerolling quite a bit. (We beat the one team we faced due to better organization.) Anyways, before the night was over, ANet had hotfixed the skill to remove the ability to exploit it like that. So the imba didn't even last 24 hours.
Also, deck build systems allow for players to create counters and balance the game themselves. This helps a lot, as does ANet's streaming technology. One thing not talked about enough is that ArenaNet has a technical advantage over about everyone else due to their streaming tech.
