Well, probably the biggest thing is it's not promised, and any returns are not being fabricated (a ponzi scheme will generally promise X% return, and will claim

Well, probably the biggest thing is it's not promised, and any returns are not being fabricated (a ponzi scheme will generally promise X% return, and will claim to have made X% on the invested money, when in reality they have made no return. When you buy bitcoin you get some bitcoin, which may or may not go up in price, but it's clear what you do and don't have).