bitflipper
I did an experiment with MSED on a reverb bus and muting/lowering the Mid. My theory was that if reverbs spread reflections across the panorama, I could filter out all but those at the outer edges, taking the reverb out of the center. I imagined that it would clarify the vocal while adding a spacious effect.
Sounds good in theory. In practice, it just made the reverb sound thin. Might be worth further experimentation, though, altering the M/S balance on sub-busses for effect. If, for example, you wanted to remove all the kick drum from a stereo drum mix in order to replace it with a sampled kick.
Trouble is I've found with increasing the ratio of 'Side' to 'Mid' (turning the mid down or increasing the side) is that the width will increase but at the expense of any depth, as you found.
It tends to work much better reducing the 'Side' component especially if you've got too much ambience from the room and want to bring more focus to the real action. Adding to the side is a surefire way of wrecking mono compatibilty with very little effort.
Slight compression deviations between 'Mid' and 'Side' are worth playing with too as are eq say some LMF removal and some added air to the 'Side' and some more weight on the bottom of the 'Mid' portion for example. Keep an eye on any added gain though so as not to disrupt the balance between the two elements.
If you are using any kind of spatial fx chorus, verbs etc. using the bus split method it's worth panning the mid hard left and the side hard right on the buses so as not to get any cross-talk between the two elements.
Those are just some guidlines I've found out by playing around but there are no limits to having fun. And you shouldn't limit yourself to mixes either as you can get really creative with individual stereo instruments/sounds.
As a technique it's not a magic bullet by any means but it's another useful option for the toolbox, and the advantage of the split bus technique is that you get exactly the same control over your elements as you would if you'd used the Mid/Side micing technique and you can use all your favourite plugs instead of investing in a bunch of specialist M/S ones.