Stereo vs Joint Stereo

One trick in order to compress the mp3 file is to encode the low frequencies mono, while keeping the high frequencies in stereo. This saves some extra bits that can be used to increase quality in other ways (most often used to fill up the frequency bands better) to the cost of damaging the stereo or "live" feeling of the music. It can be compared to playing music on a system with satellite speakers and subwoofer instead of real stereo equipment, but is normally taken a bit more to the extreme by the mp3 encoder (more frequencies being made mono).

This is a typical mid-fi solution. It's normally a good choice when encoding at low or medium bitrates since the frequency bands otherwise gets a bit empty, resulting in a worse quality degradation. At high bitrates its a bad idea.

Consequently most mp3 encoders use joint stereo for 128 kbps or less and switches to real stereo at 160 or 192 kbps. Some encoders however, especially an older version of Fraunhofer's, stuck with joint stereo even at 256 kbps

However, there are some drawbacks with joint stereo even at medium bitrates. Dolby surround information that has been encoded into the song might get damaged or lost. Music that has been mastered using analogue equipment (mostly older music, like from the 60's and 70's) and live recordings tends to suffer a lot more than modern, studio recorded music.
You can also get strange phasing effects in some rare cases with joint stereo.

We all perceive audio differently. Just the genetic and medical differences (shape of ear, hearing dips at certain frequency intervals, inability to hear higher tones etc) makes a large impact and on top of that we have trained our hearing in different ways. Some people with musical training can very precisely determine frequency variations while others might have a very good feeling for rhythm. Therefore there is no universal way of encoding audio so it fits everybody perfectly. What sounds like a perfect copy to someone might have quality glitches that someone else easily hears and the other way around.