+1
However i'd separate and limit the rewards:
1) A reward going to the player involved not the whole guild. For example every player that finish the quest receive some personal reward. The Random Reward would please most players i guess.
2) The reward that goes to the whole guild should be scaled by the number of people participating (half guild doing quest = 50% reward). And I'd put a time limit, otherwise scaling reward makes no sense. Without scaling it'll be a joke not a guild quest.
Hmm, without time limit any quest will be accepted by guildies, because some time they can meet the criteria (eg. collecting 2,000,000 coins can take them 3 months as an side-effect of loggin in 1 time a month, and something like that shouldn't be rewarded).
Guild-wide rewards, like Prestige, could lead to balance issues in my opinion but if the game scales the reward it will be very honest way to earn something.
Following my imagination in this topic, I think the quests should be pvp-neutral. For example 'Collect xxx goods' instead of 'Plunder xxx goods', so players may choose the way to finish the quest.