Eh, I'm not sure you can really compare the two.

I think when the naysaying is "this is not technically feasible or efficient/too costly/has no revenue model", yet the theoretical idea is good in a "perfect world", you have a real shot at proving them wrong.

When the naysaying is "even if this works exactly as expected, most people would not get a lot of value out of it", assuming their reasoning is correct, then the naysaying can be justified.

It looks like most of the original concerns over Dropbox from sensible and experienced people fall in the first category, not the second.

I'm not a VC or startup/business person so I could be completely wrong, but that's the view I've managed to piece together over time. I think pg's model of "can you envision a small cult following of users who will really love this product/service?" is the best way to look at it. If you can get that initial cult following, then all other problems can (likely) eventually be overcome. If you can't get that, then you're sort of wasting your time.

This is a forum for people, working in technology, hugely familiar with startups. You're going to face tough questions here, which I hope is why OP came to post.

I personally don't see the value in a service that emails a group of people and then gathers their responses when that data is probably already available and retrievable in an automated fashion.

I don't think you can compare questioning the value of Dropbox (which is a huge piece of software, handling sync across multiple platforms, even back then) to this.