The internal plastics on the slot-loaders are notorious for crumbling into dust. If you pick it up and shake it and you hear pieces moving around inside, that's probably bits of the plastic frame. Unfortunately, the outer plastics (from memory) all clip onto this one inner moulded piece that's known to rot. It won't cause the Mac to fail, unless one of the metal screws comes loose and causes a short. Also, the slot-loader's components won't fit into the much more robust tray-loader shell without a lot of surgery, which is a pity. Even opening the slottie to check the state of the plastics can cause them to crumble. Darn things.

The slotties are obviously better machines in every other sense than the original tray loaders. Faster, better video, and they take desktop RAM (as opposed to the laptop 144 pin SODIMMs in the trays), and with the exception of the very first slotties, they have a VGA plug on the back which makes re-casing them much easier (in case of failed plastics, power supply, CRT or analog board).

The Rev.A tray loader has a number of mods which can be done to it - floppy, external (Mac) video, Mac serial, ADB (iirc), second hard drive (externally, using a long 2-drive IDE cable and a Y power extender). If you find a 233MHz or 266MHz tray, it can be upgraded to 333MHz by swapping the CPU/RAM module.