Going forward from now, capacitor plague will start claiming most of the old LCs. Almost every LC board I see (pizza box or AIO) has at least started to leak capacitor fluid.
I also deal in a lot of iMac G3s. Most common problem seems to be the hard drive. I have one here where some kids literally bored out the two USB ports so now you can't plug in anything USB; the motherboard is basically ruined, unless I can somehow fashion on new USB ports.