In a basic sense, guild missions will simply be new fun things you can do with your friends/guildies. However, they must first be unlocked similar to other current guild rewards. So, I'd advise you to stock up on some influence now to unlock these as soon as possible. These are not something meant to do solo, but they will take place in the persistent world, so even if you are in a very small or solo guild, it seems like you can still participate as long as there are other people around to help.
So far, they are planning to have five(!) new types of guild missions:
Guild Bounty - you must hunt and find certain entities for the various Orders.
Guild Trek - this one is timed and seems like a scavenger hunt, visiting a bunch of listed locations.
Guild Challenge - not much info about this, but will require "multi-group, coordinated play" to succeed.
Guild Rush - an obstacle course of some sort? Making your way across traps and other dangers.
Guild Puzzle - jumping puzzles! I really hope these are not timed or I am most likely doomed. The puzzles will also require individual jumping skillz and larger, coordinated efforts.
The other new and exciting addition is that of Guild Merits. Presumably we will still get some of the normal rewards for completing these missions such as coin, karma, loot, and exp, but the guild will now also earn Merits for its accomplishments. These will be displayed in the actual guild panel and can be used to unlock new upgrades like "combo banners, gold find bonus, reduced waypoint costs and, of course, the missions themselves".
The existing guild upgrades are pretty underwhelming in my opinion. In fact, the only thing I really cared about unlocking was the guild emblem and guild bank. I really hope they are adding more upgrades than just what is listed above. *cough* Give us guild halls and player housing already!! >:( *cough* Obviously unlocking the missions themselves will be fun and the reduced waypoint costs sound nice if they are permanent, but really? More combo banners and gold find bonuses? /yawn
Guild missions do have me intrigued, however. They seem like they will help bring people back out into the world and who doesn't like more things to do in general? They still have a ways to go before the guild system truly lives up to it's potential, but this is a nice first step of sorts.
Are you looking forward to playing with your guildies in this way? Which type of guild mission looks the most fun to you? Let me know in the comment section below! :)