The expansion of the crypto markets with new altcoins and ICO's coming every day
Altcoins and ICOs serve one purpose: the creator wants to get your Bitcoins! Once you realize that, it's easy to see through most of them.
What gives all these coins their value?
The fact that everybody wants them, is what gives them value.
Maybe just a "well it's going to go up because more people want in so that they can earn money too". That's 100% the definition of a ponzi scheme
How is this different from dollars, or gold?
The dotcom bubble burst because people were expecting enormous future profits, which wasn't backed up at all by the organic growth in the industry.
You could still argue the same for many tech companies.
I expect Bitcoin to keep growing, as long as more people join. Since only a tiny fraction of the population uses it, there's a huge growth potential. All you need is adoption, value will follow. That's also why I worry less about temporary price drops now.