Irony can be such a subtle thing. This thought occurred to me whilest reading some forum posts about "What is S&W?". One of the staple answers to such a query always involves house ruling. Back in the days of 0e we house ruled. AD&D tried to do away with that. It was an attempt to cover all the bases. Not to stifle creativity, just to establish a common tongue, as it were. The idea was that if you sat down at 20 different AD&D tables at 20 different conventions, they would all be the same AD&D.
The real death knell for creative thinking in D&D was actually the internet. There have always been plenty of people willing to step up and create new classes, subclasses, systems for this or that, and so on and so forth. Back in the day, you had to wait for the next issue of Dragon to access these folks, and they never directly addressed your needs. You took what was on offer or you didn't. Then came the internet.
Consider this: In 1977 if you wanted a different magic system, you made it yourself. In 2007, you Google it. There are plenty of people who have already done the work. If you don't like one, there're plenty more. Their work is still considered house rules, but not your house. DMs (I prefer the original term, Referee) became lazy. Since the advent of AD&D they had been discouraged from deviating from the written word. Now of the intrepid few that did, many of them just used the work of the other intrepid few.
Then, as irony would have it, the internet came to the rescue. Games like S&W, Labyrinth Lord, and BFRPG sprang up. Not only do they encourage individual referee's to house rule and customize, they require it. And, thanks to the internet, they are in wide distribution among players who share their passion for such gaming on forums and across the blogosphere.
Long live the clones and the intellectual exploration they engender!