Gotta go with Scott, here...
The first thing to go with low bitrate mp3's is the cymbals, and any sense of 'openness' when heard in stereo, IMO. Just TOO grainy and undynamic for me. My iTunes library is ripped at 192kbps, about the minimum I can stand, but I'm starting to wish I had gone for 256 now.
But one GOOD thing about using lower bitrate MP3's is... it makes what you play stand out front and center, it is SO much clearer than the backing!
If you CAN'T hear the difference between a high bitrate or the original and a 128 MP3, time to either get better speakers, or off to a hearing professional (none of us getting any younger!)
Just do a simple A/B sometime between CD and MP3, you'd be surprised. It's subtle, but for me, the MP3s induce ear fatigue a LOT faster than uncompressed...