Per-track ReplayGain shouldn't be enabled by default
It can cause unintended loudness jumps between tracks on the same album, which is a big problem for albums with seamless track transitions.
On Pink Floyd's album "The Dark Side of the Moon" I get an extreme 17dB loudness decrease between "Speak To Me" and "Breath" when per-track ReplayGain is enabled, which is how I noticed this issue. I at first thought there was something wrong with my audio files, or with Audacious, then I remembered about ReplayGain. Someone who doesn't know about ReplayGain will probably just uninstall Audacious and avoid using it in the future.
Per-album ReplayGain should be the default. With the exception of badly mastered compilation albums it is generally preferable anyway, since not all tracks are meant to have the same overall loudness.