It would be nice to be able to merge Opportunities or move them from one Account to another. 

We often have the problem that Company A calls us for a system and we open an Opportunity with them.  Then Company B calls us for the same system and we realize that both are bidding on a system for Company C.  If we already have two Opps opened at Company A and Company B we'd like to merge them, and put them all under Company C since that's where the equipment will eventually end up.  Although we may not have direct contact with Company C now, they will be the ones likely calling in the future for spare parts, service, etc. so we really care more about them than the other two and feel the Opportunity really belongs there.