Everyone who listens to MP3s knows (or should know) that MP3 is a lossy medium; in order to get a file that's only about 10% the size of the uncompressed music, some frequencies are thrown out based on how well the human ear hears them. You just don't get as good a sound from MP3 as from the source CD.
But wait!
It turns out that the sound engineers producing those CDs are now using MP3-through-iPod-earbuds as the reference for how the music sounds. And when there are things that sound great in the original CD-quality music that they don't hear or doesn't sound as good on the iPod, they'll cut them out!
The reason is that these days the music is considered most likely to be heard on an iPod, so that's what it's optimized for. It reminds me of the cell phone catch-22 of the past 5-8 years: As more people got cell phones, fewer used pay phones, so pay phones started disappearing, forcing more people to get cell phones, perpetuating the cycle until there are hardly any pay phones left. Similarly, as more music is heard on reduced-quality MP3, more music is optimized for MP3, prompting more people to listen to that music where it sounds best, forcing even more music to be produced for that lower-quality medium.
I do *like* the convenience of MP3, but I'd still like to have the better sound of the CD available! Besides, I probably encode MP3s at a higher quality than most people.
Now I dread the day when music production is optimized for hearing as a ringtone.....