I'm curious what older lines are actually of interest.
I began collecting some of the older lines largely on account of the lack of structures in the Halo line. I began with structure-heavy lines like Command Ops and Alien Agency, but branched out to Pirates of the Caribbean, Probuilders, Marvel (Spiderman and Fantastic 4 primarily), Teenage Mutant Ninje Turtles, and Xtreme Sports. Many of the sets I elected to collect either had the right parts in the right color, or other alluring elements like some of the fabulous trees MEGA used to make.
However, I've become particularly fond of the Alien Agency line. The agents and physician figures are actually pretty nice.
What old lines have you collected and why have you collected them?