One thing that worries me personally about starting the "stereotypical" HN startup right now, is that I think the free/fremium model is going to lose steam FAST. My prediction is that you will see more companies trying to start monetizing their web-app soon. FWIW, TipJoy is well positioned to take advantage of this if my prediction is true.
Of course, I've always been a hardware/product startup hacker, but I've had a couple of rounds in software and Web 2.0 companies. So, I'm not as immersed in web-app funding cycles as some others here are.
I would look to solar and certain other viable alternative energy options right now as the next major tech growth sector. I think the tech startup world needs to see more "inventions" (again, admitting my own bias).
While many here may disagree with me, one of the problems with web-apps is that while they are cheap to get going (relatively speaking), I think that the saturation of non-revenue producing web-app companies, coupled with the low cost of starting them, will make many traditional VCs look elsewhere for their next investments. The web-app community needs more investors like YC, that keep the investments and process realistic.