Thoughts
Gemini's mistake, if it made one, was being radical in too many different ways:
* It was radical in having presentation controlled entirely by the client and having documents be fully semantic
* It was radical in being very simple, non-extensible, not valuing growth, not valuing rapid development
* It was radical in encouraging client and server development ("making a client/server is easy" was a design goal; that's not true for any other protocol or specification I can think of)
* It was radical in encouraging TOFU certificates