The websites of all the YC companies look similar. Have they all been worked on by the same designer?

That is encouraging. If folks with no previous design experience (assuming) can dish out beautiful html and css, that is indeed encouraging to the rest of us :) Any pointers to templates or design guides?

It does look very similar to the YC sites, but that's not a bad thing at all - it looks very good, intuitive and beautiful.

Very, very nicely done.

There's probably some truth to this, but on the other hand, there are some good UI reasons for following conventions. Graphical conventions can make your application easier to use, because people already know them. The world of UI design is fulll of such conventions: red: stop, green: go, squares: check multiple, circles: check one. US light switches are easier to use than the European kind because they are more consistently mounted in the same direction. Designing UIs is all about finding metaphors that already make sense to people, either because they are used in the real world, or because they are used in some very popular systems people are already familiar with. (E.g., most desktop applications put "find" in the "edit" menu. Does it make sense? Not really. But by now, that's where people look, so that's where you should stick it.)

That said, there are some conventions in recently fashionable web applications that seem to be nothing but fashion. That's less of a UI design thing, and more of a marketing thing. Presumably, there is some sweet spot between total conformism and utter disregard for the contemporary norm that strikes the balance between attention-grabbing and comfort-instilling. Personally, I would say: when in doubt, keep it simple. (That's my personal taste speaking, not any kind of business experience.)