I really like the idea of having some way of determining who needs/wants to get certain missions and quests done from most major game areas. However, I think the system you are describing, while certainly well thought out, would never be picked up due to it requiring a lot of effort to implement. You say the system is best implemented through the auction houses (presumably because you can access them from all major locations), but this would require changing the way players interact with the auction house, there would have to be some new system for selecting whether you wanted the Board or the AH. You could just add NPCs or objects near the AHs to remedy this but now you're taking up more system resources by adding more models to the area. You're also talking about adding a "Points" system to the Board (I'm assuming this is to both prevent spam and make sure people actually get help), but the issue is how people check this. Is it through a new NPC or is it a menu item? I also don't really see an issue of people not helping, forcing people to group up artificially might have further repercussions (I don't know if this would really be an issue, it's just something to consider, the social consequences of creating this system are not for me to talk about). These points also require storing more information for each player. I also think that the board itself might require the creation of new data (you've stated above that it could just access existing data, maybe you are more well versed in the technical aspects of the game than I am). A Board "Post" must now exist somewhere containing information (or in the least the location of information) such as who is requesting, how many people have already signed up, what each reward would be (points, gil, item), the cost, what the quest or mission is, and probably a text description written by the poster. Finally the whole reward system you describe seems to me like it would throw off the balance of the game (maybe I've misunderstood you). It seems like you're suggesting that if I requested a party for to kill an xNM for x item like minded players could also receive said item, even though only one dropped, as a reward for helping (again maybe I'm misunderstanding). That would then devalue items or require them to be re-balanced perhaps ruining their beneficial nature. It also seems like it might be fairly complicated to figure out for instance if a request of that nature had been completed. To sum all that up for you: I really like the idea, but I think it would be difficult to implement at a technical level and have a lot of unforeseen repercussions.
I'm not just going to nitpick you though, I think this feature could ultimately be implemented in much simpler ways. For instance it seems to me that another search function could be added. The server already knows what quests I have active and what missions I am on. I think it would be a lot easier to add a mission/quest search feature. This would (at least as I imagine it) allow you to search for players in an area (or the whole server or whatever) who have the selected quest/mission active. I think really the biggest issue with this is that the game would have to either present you with a full list of missions and quests (which is clearly unreasonable) or would require the game to keep track of all the quests you have active and present you with that list (which may require creating more data or menus etc). I suppose another option would be requiring the quest name to be input as the search term (like the search name feature). I suppose it could also be pointed out that unlike your suggestion I don't really provide an incentive for people to help, but I've never had an issue with the community's willingness to help out. I think the biggest plus of doing something more of this nature is that it uses existing systems, menus, and commands.
Anyway, if I've misunderstood your post please feel free to correct me. If you have a better understanding of some of the underlying technical elements of the game, feel free to correct me on that too. Like I said at the start this is the kind of feature that I'd like to see exist.
:D