1. laserdisc mastering defects (e.g. interlaced frames, trails)
2. film defects (scratches, cig burns)
3. special effects shortcomings (matte boxes)
4. bloopers (continuity)
My own opinion is that 1 & 2 should definitely be addressed, 3 is arguable and 4 is only for a Darth-Editous-type Special Edition project.