Also, the fact that Dropbox and Facebook were "unoriginal" is exactly what made them seem like bad ideas at the time. I saw Dropbox present at demoday, and the main question on my mind was, "How is this different from the million other attempts at online file storage? (xdrive, etc)".
It seems to me that both Dropbox & Facebook are example of companies pursuing good ideas that lots of other companies were also pursuing. They just happened to be the ones that executed the best (and possibly also had the best luck) so they were able to win the market.
If this is the case then your job as a funder of startups might be to try to identify the companies capable of out executing everyone else on a good idea that a lot of people are working on.
This is much different than the great idea that looks bad phenomenon that pg discusses in his essay.
I don't have the time though, so if any bloggers/journalists are reading, this could be an interesting article, or even book, if you get enough info.
Whether or not this info would actually help another team become successful is another story though.
The big successes that I've seen all have exceptional founders, but that's clearly not enough. Timing is a huge component as well. Often a good idea will fail simply because the market or technology isn't ready. My own experience with Gmail reflects some of that. When we decided to write the whole frontend in JS, everyone said that it was a bad idea doomed to failure. It had been tried before (e.g. desktop.com) and had always been a disaster. They were right about the past, but wrong about the future. We released Gmail right around the time that browsers were finally getting good enough, and were were very careful to keep our code as fast and slim as possible.
The actual formula that academics use is Performance = AMO, ability * motivation * opportunity. I think there are a couple of modified versions but I can't remember what they are off the top of my head.
This is to say that my experience confirms that there probably is some completely subjective formula that will fool people into thinking there is some reason behind my good luck. :-)