A good box set should either be
a) 100% non album tracks. outtakes or b-sides or live or whatever. and be comprehensive in collecting all the stuff that's out there. Also sequence the songs so that you aren't listening to slightly different versions of the same song 5 times in a row. I hate that. (only exception is with "sessions" sets which are more of a historical document of a song's recording process than a listening experience).
or b) if you're going to include previously released stuff include everything. All the albums packaged together for a complete discography set. or if the band has too many albums have separate boxsets that each compile a specific era with no overlap between them.