Coming from an almost vanilla player (since TBC) I think the most active I have seen guilds was when they were working on guild achievements, guild rep, and guild level, etc.
I think it is time we up the guild level cap and add a few 'useful' benefits as level rewards. This not only highly promotes social gameplay but it also puts some loyality and ownership of the guild to each member. I remember many of nights just fishing in groups or a indepth supply process to get mats to create high ilvl items or potions, or even just go fishing for the evening. Since we capped, everything has gone back to stale members with no need to run guild groups or even keep ppl from hoppin level 25 guilds (since it is so easy to get guild rep now.)
At least maybe a 5 level increase for each major update or something would be nice.
Thanks for letting me bend an ear.