Just wanted to throw out there that I've heard a couple examples of this where there was only a slight audible difference between the original and the mastered version. But most of the time it seems that it's the slight differences that are cleaned up by the ME.
I actually just got back from the studio where I had the opportunity to listen to a pre-mastered and final radio mastered version. Some friends of mine are working with Tree Studios here in ATL to get their latest single ready for radio. In their particular song, the biggest differences I could pick out immediately was that:
- guitars were pulled out and given a lot more punch
- vocals were cranked up
- the low end was boosted
- they cleaned up the drums (cymbols/hat hits) with a sampled kit
- they brought in extras to do some overdubbing on the lead vocals to give it more presence
- they clipped the intro to bring the song to under 3:20
- compressed the hell out of it (can't really hear it, but you can definitely see it when you load the waveform into an editor)
- eq'd throughout
Think that's about it (I'm sure there's tons more that I couldn't see or hear that was done to it). Pretty interesting to learn what these big studios do to music