[very common; always pronounced as if capitalized. Orig. fr. the 1930 Sellar & Yeatman parody of British history 1066 And All That, but well-established among hackers in the U.S. as well.]
1. Self-evidently wonderful to anyone in a position to notice: “A language that manages dynamic memory automatically for you is a Good Thing.”
2. Something that can't possibly have any ill side-effects and may save considerable grief later: “Removing the self-modifying code from that shared library would be a Good Thing.”
3. When said of software tools or libraries, as in “YACC is a Good Thing”, specifically connotes that the thing has drastically reduced a programmer's work load. Oppose Bad Thing.