This may have been brought up in the past but I thought I would bring it to light and see what others think.
The way things work right now, you can be with a guild for a while and earn 1 million contrib XP for the guild. If a friend needs your help in another guild, when you leave the guild looses all of that XP. When you return, the XP still shows in Guild Adviser but your progress goes back to 0 and the Guild does not get any of the XP back.
I am fine with the player's personal progress being reset but I think the Guild XP should reflect the total in Adviser.
Alternately, if a player has been in a guild for a very long time but then decides to leave, why can't the guild keep a portion of the experience? (not sure if I like this idea but just throwing it out there)
One big issue has always been if a very high level player joins a guild for a few months and racks up a LOT of XP for that guild but then decides to leave, this can drop the guild many levels in an instant and leave that guild in a big hole to try to get back to where they were before. I have also recently seen one of the top guilds drop multiple levels when one of the top contributing players was banned. Regardless of what the player did or didn't do, the Guild paid the price there.