For those that just got rejected: Welcome. If you're seriously getting into the startup world that will be your first of dozens of rejections. Learn to take it in stride.
Investors invest in stuff they get, stuff that's presented well, and stuff that they relate to. That's triply true for early phase where they're going more on a hunch than anything even bothering to be hand-wavey with projections.
Just one kind of cute story, both from successful founders cum angels: We've had meetings with two angels now. One of them thought my co-founder was a turd. The other thought he was the swellest guy on the planet and has been telling all his buddies about us. Are either of them idiots? No. In one case things clicked, and in the other they didn't. All of the business plans and prototypes in the world wouldn't have changed that.
All early phase investors make some dumb investments. In fact, most early stage investments are dumb. YC is a bit ahead of the curve there, but they don't have a crystal ball.
news site: http://reddit.com
geolocation on cellphones: http://loopt.com
form builder: http://wufoo.com
server administration: http://virtualmin.com
electronic components: http://octopart.com
Etc, etc. Lack of diversity? It seems like these ideas span the whole range of Internet startups.
You realize the whole "Clone Amazon for me, $100-500" thing doesn't actually work, right? And look at the last batch of YC companies, will you? There's a lot of diversity there.
Maybe the only thing you got right here is that some of the start-ups YC doesn't fund will likely go on to succeed. Since YC themselves admit that, it wasn't much of a point though, was it?
Ycombinator definitely has some diverse ideas many of which are fantastic. Like any start-up catalyst, sure, they have backed some ideas that have already been out there to some degree or are no longer around--but that is totally to be expected. There will always be some overlap and like any regular distribution bell curve will show us, some will fail, some will surpass our expectations and rest will be in the middle. I have a great deal of respect for all that Ycombinator does and the people involved. Good luck guys!