OK. Now think back to the days when you used to watch pro wrestling. Remember the tag team matches. Now, remember what happened about 90% of the time when one wrestler left the ring and tagged in his partner... chances are, for a few seconds, they broke the rules and double teamed their opponent. Most tag teams had some kind of special move that they used as a pair when the tag took place; they had a tag team combo attack. Now... imagine for yourself if you could bring something like that into your game of 4e D&D. I have and I think It would be really, really cool.
Now, in D&D most of the time, there never is much of a tag. You are pretty much double teaming your opponent from the start. But, what if there were powers that party members could use together, rather than individually. I think it would be both mechanically cool and provide an awesome story-based element to have powers like this.
In terms of the story element. Tag team powers would give your players an element which ties the characters together. There are some feats now that provide this kind of story based tie-in between the characters now (so there is already a precedent for mechanical options which are only useful when more than one character takes that character option). These powers would go beyond just working cooperatively, i.e. one character holds the monster while the other person stabs him repeatedly; these powers would be like character trademarks. Two characters in a party have a signature move that they always do as a combo. They never do that combo apart, it is a combat maneuver that is unique to their tag team. That to me seems like a great way to link two characters together in terms of their background.
Mechanically speaking, it would not be hard to implement either. Two character take the same feat. That feat offers these two characters a single encounter power which they can use together. To use one of these powers, the member of the pair highest in the initiative order delays his attack action until his until his partners initiative pass. Then, the pair execute a single encounter power on the same initiative pass. This single uber move, is a little bit more powerful than two different encounter powers separately for the level (making up for the fact that two powers get used and one character has to delay).
I think this would be a pretty good addition to D&D 4e and I invite you to join me in writing up some of these feat granting tag team powers. I will be posting many of these in days to come.