For me, balance is fun. I come from Ragnarok Online, which had a huge degree of customization, but in the end it came down to exactly what other people who have played other MMOs with extensive customization have been saying: certain builds are deemed to be the best and then people either follow those builds or get given grief for not doing so.

Which is fine if you're only interested in doing solo content, but not much fun at all if you want to play in groups. The customization process itself wasn't fun, either: I spent literally hours on end grinding mobs to get the right cards to put on my gear so that I would have good stats and abilities.

There's no reason to add skill trees or different specs to a game that already allows you to play all of the classes on one character. If you want variety, you can simply pick up a different class-- you're not stuck with one thing. There's even a certain amount of customization you can already do with the classes themselves (VIT vs. STR on tanks, the examples other people have given for healers, etc.). Adding more stats/specs would be unnecessarily complicated and would bring no real benefit in exchange.