1) Is Bitcoin environmentally sustainable? (Argument: the amount of energy required for computing power in mining is inordinately high).
2) If it does not transition from being an asset speculated on to being used primarily as a currency, does that make it akin to a pyramid scheme or ponzi scheme with nobody in charge?
3) Are miners "painting the tape"?
4) What is the best criticism you have heard made against Bitcoin?
5) As it has not experienced a full business cycle before, what is most likely to happen to Bitcoin once the recession/depression that is around the corner finally hits?
1. It's difficult to predict. The amount of energy consumed depends on the cost of the energy and the value of a bitcoin.
2. Yes, but there is no such thing as a "ponzi scheme with nobody in charge".
3. There is definitely a problem with exchanges reporting and generating fake volume, but that doesn't have anything to do with miners.
4. It is hard to use and not widely accepted.
5. It depends on the nature of the downturn, but I think Bitcoin will behave like gold.