Quick thoughts:
Having a portal for Guild Missions is both awesome and convenient. I still probably won't do any but still...
GvG arena confirmed. Arena can also be used to privately duel, although it'll be with PvE armor instead of PvP armor.
More WvW integration. Guild Hall is also going to be linked to PvP team leaderboards.
New crafting profession for Guild Halls. I wonder if it's related to those motorbikes...
My only concern is that you assault the Guild Hall in order to capture it. That's a lot of work for a one time thing. So I have to wonder if there's a game mode or Guild mission attached to it, maybe as a defense mission. If true, it's likely not PvP focused, but I'd probably have some fun with it. I really enjoy horde modes in certain games (Halo's Firefights being one of my faves, Call of Duty Nazi Zombies being one of my least favorite).
I'll wait to buy right before the first beta weekend.