I always thought that "Those who can - do; those who can't - complain." was said or at least attributed to Linus Torvalds. However, I could not find any place in Google with this search. Then I looked for it in Google Groups and as the oldest found this post to comp.emacs by an obscure "Jonn Beudin" where he says it in 14 August 1989, three years before the Linux kernel effort started and Linus Torvalds became famous and started posting.
The post reads:
I think you owe the implementors of emacs an appology. You have very clearly not made an attempt to read the documentation. Instead you flame. Please be more considerate in the future. I feel bad when I read about freeloaders (:-) like yourself cutting up something they didn't contribute to. I use emacs and am damn glad some people had the generosity and sense to post it. "Those who can, do; those who can't, complain."
So it wasn't Linus originally after all. I wonder if the quotes there are meant to quote another source, or are the "It's an aphorism" indicator. So it may possibly be a meme.
I found many other variations on the "He who can, does. He who cannot, teaches" on Google, some of them Slashdot related. However, the Shaw original is not true, as teaching requires even more effort to do properly than doing something productive. The Talmud says "I learned a lot from my teachers, more from my friends, and from my students, the most of all." Teaching is the best way of learning, and I'm speaking from experience.