Starting a cult whose core teaching is essentially "Think for yourself, schmuck!" together with techniques for doing so effectively may be the worst idea for a cult in world history.

SilasBarta has already pointed out the obvious counterexample; a variety of other vaguely cultish institutions, such as Anton LaVey's Church of Satan, also share goals that are, if not identical, then certainly within spitting distance. More importantly, though, I don't think "think for yourself, schmuck" is actually a core teaching of LW; LW-rationality is aimed at achieving success (however defined) rather than maintaining some ideal of independent thought, and it'll happily discourage any ideas, however independent, that its group consensus agrees are maladaptive. This isn't even a bad thing; independence for its own sake is best left to the Discordians.

I don't think LW is a cult or in serious danger of becoming one in the near term. But its core ideals alone are not sufficient to ensure that it doesn't have the abstract potential to become one.