Keep in mind this is just me musing about what would be a nice feature in digital TVs (and converter boxes), not something that actually exists, so far as I know. The thread on how commercials are inevitably louder reminded me of another common problem I've noticed: TV stations have grossly different volume levels. I crank it up for channel 65, then switch to 7 and the damn speaker nearly burns out.
So what would be nice would be a "volume normalization" function. Could work lots of ways: easiest to implement, but most difficult for users, would be a setup option that let you adjust the *relative* volume level for each channel individually (probably just a simple +/- slider control). When you tuned to that channel, it would retrieve this factor and apply it to the current volume setting.
Better for the user, but harder to implement would be an auto-normalization function that would run when you do a scan (or rescan) of channels; it would take a short sample (say 5-10 seconds) of each channel's audio and automatically calculate the normalization factor, then store it. Of course, this would take a lot longer, and it's not guaranteed that the sample time would be representative of that channel's sound level.
This does nothing to alleviate the annoyance of loud commercials; it would simply be a way of "leveling out" the sound level of TV stations without requiring legislation, FCC regulation, etc.