I loved the sleep light; you always knew if it was safe to turn off the mains electricity e.g. for the night. On the newer iMacs you can't tell if the computer's asleep or if someone's already turned it off. (Cue hit keys on keyboard, wait, see if any response after a few seconds cos it doesn't always wake up completely immediately).
It was a nice soothing pulsation too, and turned off when the screen was on. Apple really thought about that LED. Everywhere else there are/were superbright blue LEDs flashing and screaming at you while you tried to work.
My parents love my old iMac G5 and we miss it! Hope to get it back for the spare room if/when I upgrade my current Alu-iMac.