Guilds aren’t a new feature for WoW, and we are really sensitive about imposing any kind of structure on them that might make you feel like you aren’t supposed to stay with your current guildies any longer. To that end, there were flaws with guild talents that we didn’t like.

Guilds are, for the most part, groups of friends. We don’t want features to drive wedges between them. We don’t want you to guild hop looking for the talents that suit you best; we want you to play (or in many cases keep playing) with your friends. With a talent tree, guilds would naturally have different talent trees, which creates a reason to bail or guild hop that doesn’t exist today. If you didn’t like the direction the guild was taking (for example, you were a PvP guy with a GM focused on raid-progression) you might feel like you should leave. That can happen today of course, but you can also stay in a PvE guild just because you like the members even if you don’t participate in the same content because you’re not losing out on inherent guild benefits.

Furthermore, we felt like the decision-making, for many guilds, would be up to a relatively few people, possibly as few as the guild master. Talent trees work for classes because the decision is up to you. We didn’t want to create the risk of drama over choosing those talents or even not being consulted in choosing them.

Another advantage of the perks system is that it’s easily expandable. If we want to add new levels and / or perks in later patches or expansions, we can, without having to rejuggle a talent tree. (Source)