(This might venture into the land of feature creep. I apologize in advance)
Currently when viewing replays, the entire map is displayed to make it possible to follow what the various players do. What I would like to have as an alternative would be to assume the view point of a certain player. This could be useful when watching replays to see what the player has seen, to better understand the choices the player does. Compared to viewing the entire map, this would make it possible to know what the player has and has not seen which can be useful to figure out why the player did things such as build a specific building at a certain time, or attack at a certain place.
I think this could be implemented by having a drop down list (though that would require someone to actually implement one first, see bug 536489). From the list it would be possible to choose between all, player1,player2, etc.
Alternatively it could be possible to tie the number keys to the various players (0 for all). I realize this would conflict with the stored locations, but I am not sure whether they make a lot of sense. (edit: when viewing replays. I use the stored locations all the time when playing the game, but I don't really see the need for them when watching a replay. Then again it might be slightly confusing that the number keys suddenly do something else.)
I am aware this can be considered feature creep, and it is certainly not a big issue currently. Actually, even with the feature, I would not know of a lot of replays where it would be useful. However, with tournaments such as last year and other multiplayer games it could be cool.
I agree that this is a cool feature.