HD quality is important, but there are definitely other factors. I remember way back when NCIS started I wanted to icon the show so badly, but the early seasons had this truly awful orange-y tint to it that made it nearly impossible (well, at least with my then-knowledge of icon making) to icon. (Check out this recap of a season 1 episode. Caps like that, even if they were HD, would require a lot of colour correction.)