Width or Depth
I agree with "Mastering house", I think many people tend to mistake lack of depth for lack of width.
There are so many factors that come into play with a mix. For instance; monitoring can play a huge part in the way you perceive the mix. Having diffusers on the rear wall facing your speakers can improve the stereo image you hear. If you also place acoustic foam absorbers on the walls at each side of your speakers, you eliminate side reflections which can cause cancellation of certain frequencies, and again change the way you hear the stereo image. So what I'm saying is that room acoustics play a huge part in the way people mix their music.
If you think about it, say you are listening to an
acoustic guitar right up close, the sound takes up the whole stereo field of hearing. As the guitar moves away you can pin point its location more accuratetly because it is closer to one ear than to the other, so the direct sound from the guitar doesn't take up the whole stereo field, it seems to be coming from a particular place.
What happens as it moves away, is that the direct sound dies away, and the ambient sound which is reflected off the walls becomes more apparent. So your brain interprets this as the sound moving away from you. At this point the sound is less intimate but more ambient, it seems to have more depth (and width!).
As a sound moves further away, the treble dies away faster than the bass frequencies, so the sound becomes less clear. This is the case when using reverb; a really distant sound has very little high frequency content in the reverberated signal reaching your ears.
In terms of dynamics, distant sounds are dynamically flat; ie, very compressed, whereas close up sounds are uncompressed and have strongly varying dynamics.
So you can seriously widen! your mix by knowing these things and giving each instrument a 3 dimensional space to inhabit (as in real life, rather than a 2 D space by just using the pan)
Again, the above explanation is a very simplified view of what happens, but should give you an idea of the things to look out for when trying to get more depth and ultimately more width in your mix.
Cheers
Theo C
http://www.theoc.co.nz