File size and popularity are probably the most serious reasons against flac. I hope Amazon will switch their music downloads to flac because storage and data transmission costs are getting cheaper and so file size won't be a barrier anymore in the near future. MP3 is just more popular until now and supported by most devices, but with that lossy encoding you can hear the difference in blind tests very often (depending on the bitrate of course). Also even with a high mp3 bitrate, the difference may be inaudible but there is still something missing and that could effect the general feeling of the listening experience subconsciously. I want to avoid that, even if I don't notice it in a blind test.