The big difference is that with dot-coms the scams and shady characters were in a minority, whereas with blockchain they are the vast majority (so much so that it is hard to separate the signal, if there is indeed any, from the noise). Don't think anyone would debate that point. Another (albeit more debatable) difference is that dot-coms were pretty focussed on users, while blockchain development seems to put very little genuine thought into how ordinary people would benefit from any of it - it is mostly about "building out the ecosystem" (which sounds suspiciously like "recruiting new members to our multi-level marketing scheme"). In my view these differences are because the dot-coms were driven primarily by the technology and the exciting new things you could do with it, while blockchain development is driven primarily by money (to make matters worse, psuedo anonymised money which was originally used almost exclusively for illicit purposes) hence bringing out the worst elements of human nature.