While lean is generally a great step forward implementation can be quite tricky.
Saw a video last night by Jeff Lawson of Twilio about a lean test he conducted on an email sending API. It's starts at the 9:30 mark:
He did a very thorough test using Google to drive traffic to his test website along with various A/B tests on pricing.
His conclusion was there wasn't a viable market. Tell that to SendGrid and their various competitors ;<). I've done the very same thing with several sites and was always left wondering was it the test or my idea?
My conclusion is that to do lean successfully you also need to get off the web and get out and talk with potential customers face to face. Sometimes if you listen carefully they will hand you an even better idea.