I do not know whether this has already been mentioned elsewhere for vB5, but I've been looking all over the place for a vB4 modification that would allow users to be notified if someone quoted one of their post. In my opinion, having this feature built-in would be a huge game-changer and will help vB5 communities increase their user interaction/participation %. Slickdeals.net (for example) has taken huge advantage of this system, giving the users a pop-up in the bottom left corner of the screen when they refresh or visit a new page (IF they were quoted). The notification contains thread title, username of the person who quoted you and a direct link to said quotation.