This mechanic has always annoyed me a little bit and with the rewrite of everything that is coming in Cataclysm now it the best chance to get them right.
For example this new holy power mechanic that is being added to Paladins is to some extent how combo points need to work. The combo points for Rogues and Feral Druids needs to accumulate on the character and not the target. This will make for smoother game play in the end. Also to prevent the accumulation of a full 5 combo points, waiting a little bit and then opening with a large finisher on a different target put a gradual decay on either an individual combo point or all current stacks of combo points.
Individual Combo Point Decay
After X# of seconds 1 combo point decays, most likely longer than 2 seconds but less than 6 seconds so a full stack of 5 takes about 30 seconds to decay.
Timer is reset by melee attacks, like divine plea.
All Stack Decay
After X# of seconds all combo points decay, most likely somewhere between 15-30 seconds for this. It needs to be long enough that while dodging AoE in boss fights they will not loose their stack if they are careful. However it will prevent them from hoarding a large amount at the end of trash to open up on a boss or new PvP target with.
Actually for the same reasons as listed above the new Holy Power mechanic for Paladins will also need a decay like what I am suggesting here for Combo Points. The main reason I think this needs fixed for Rogues and Feral Druids is the new ability Rogues are getting to shift all combo points to a new target but not giving something like that to feral druids that use the same resource and mechanic. If it is good enough to rig a pseudo fix for one class then it is good enough for all that share the same core mechanic or just fix what is broken.