Gapless playback
Playing tracks back to back without a single sample of silence between them — taken for granted on CD, surprisingly hard with compressed files.
The trouble is that MP3 and AAC encode audio in fixed-size frames: 1,152 samples for MP3. The encoder pads the start with silence to cover its own startup delay and pads the end to fill the last frame. Play the files naively and every transition gets a tiny hiccup, which ruins live albums, classical recordings and every record where the songs run into each other.
The fix is metadata stating exactly how many samples to discard. LAME wrote it into its own header, and Apple added the equivalent for AAC when iTunes 7 gained gapless support in 2006. FLAC and WAV never had the problem, since they store precisely the number of samples that were recorded.