(From the 1930 Sellar & Yeatman parody "1066 And All That") Something that can't possibly result in improvement of the subject. This term is always capitalised, as in "Replacing all of the 9600-baud modems with bicycle couriers would be a Bad Thing".
Opposite: Good Thing.
British correspondents confirm that Bad Thing and Good Thing (and probably therefore Right Thing and Wrong Thing) come from the book referenced in the etymology, which discusses rulers who were Good Kings but Bad Things. This has apparently created a mainstream idiom on the British side of the pond.