It's pretty hard to find any specific information on the Apple III Plus. How did it differ from the original Apple III? I assume it didn't have those infamous heat-related reliability problems? Anything else?
Edit: To be more specific, are there any ROM differences? Any differences in the memory map, new or changed soft switches, new hardware registers? Any differences in the Apple II emulation?
In short, are there any software-visible changes?
Regarding added hardware such as the clock mentioned in the answer, is it directly compatible to stuff that was available as a plug in card for the older model? Are there fewer slots because of it?
And: is there any material online regarding any of those changes?