After a lengthy discussion in our guild, which was spurned on due to a senior guild officer leaving the guild, I have a few suggestions to make. (Just read the bold stuff if you don't want to read the whole thing...heh)

1. Guild Quests for higher levels are a pain, repetitive and restrict game play
2. Guild coins are fairly useless so no real incentive is there to acquire them for higher level players
3. Guild activities needed

1. Guild Quests
The complete so and so adv for high level players really restricts their ability to play the adventures they want to play. They also seem to be the same old tired quests. The member that left our guild made screen shots of every guild quest for 2 weeks, and they were the same handful of quests. He loved the social aspect of the guild, but hated the quests. It didn't leave him the luxury of doing the adventures he wanted to do. 2 options I can think to remedy this...

a. Make guild quests opt-in and use the opt-in number as the percent of active players that need to complete quests.
b. Remove the specific adv type quests so that high level players are not forced to adhere to specific adv completions

2. Guild Coins
While guild coins seem fairly useful to mid level players, they appear to be totally useless to high level players. Some easy solutions below.

a. Add more things for high levels to Guild Market, they can even be expensive to buy. Paying thousands of guild coins for certain items would still be useful
b. The ability to exchange the guild coins for things in the provision house (but it would need to be cheaper than simply selling the guild coins for gold coins and then purchasing the item in TO)

2. Guild Activities
Socializing is a big aspect of guilds and I can't help but wonder if there are ways to encourage this more. Stuff like guild collectible hunts (new guild quest perhaps) where guild members can join a new island and hunt for collectibles. A possibility for things like guild banks, where donated goods can be requested by members, or maybe even those guild quests can ask for resources for later distribution. Haven't really thought his one through so feel free to suggest things.

Final Comments
After lots of discussions the whole guild concept seems to be geared to mid level players. I believe a lot of work needs to be done to keep the high levels interested in being part of guilds and creating better incentives for them without the drain the guild quests create for them. Doing something like nords and then getting a measly 25+100 or so guild coins is Lastly I am mid level myself and don't find the problems that higher levels seem to experience. But I can see their point of view and it was a big loss to our guild losing one of most loved officers due to dodgy guild quests.