Atl coins that provide no new value over bitcoin, but are simple copies, IMHO are the biggest threat to bitcoin in general. I have a ready response to all the doubters questions except for the proliferation of alt coins...
The only threat to Bitcoin is a better currency. If Bitcoin is indeed "better", let it prove it over the course of history. Anything else is subjective BS. What matters is that math based currencies find good roots. The "brand" name is irrelevant to solving global monetary issues.