It would be a nice to have as i find there is a lot of volume variation especially with older tracks but it is difficult to pick up on PC. i have tried an number of software like MP3 gain and others but they all either worked with some and not others or pumped them up too loud so the were distorted.
i find that the differences are most noticeable in the car which is where i play a lot of my music.
On Hi Fi equipment it can be harder to detect

Definitely would be nice. Right now I'm using a batch script I found either on some random blog or a gist that runs metaflac over the flac files and applies replaygain tags. Which is fine right now, as I'm literally re-ripping my entire library, after one hard drive failed, and the other backup was in a computer that was stolen. Right now I'm using fre-ac, sound-juicer(when the CD isn't in fre-ac's cddb entries[hopefully MusicBrainz support gets added soon to fre-ac as the developer says it's now moved up in priority with freedb having gone away]) as sound-juicer is way slower than fre-ac. In the long run it would be nice to rip and forget in the future when I'm just adding new CD's.

Replay gain support is a must have.

Note that the R128 algorithm/method is an emerging standard in this space and should probably be implemented too!

Nice feature but pretty complicated to implement. Use dbpoweramp non destructive ReplayGain feature (implemented as a "coded") for both track and album in the meantime.

It would be nice if there was a rule to at least identify files that are missing the REPLAYGAIN tags even if there was no mechanism to calculate and add them.

Or at least a requirement for the tags like genre and year. (I use track and not album)

I definitely would like this, automatically applying gain to mp3, flac or ogg files,