ideas are cheap, implementation is hard.
I am very competent web app developer, with several major applications under my belt. While on the surface you might think you will be able to whip out a clone of one of the popular web apps easily (let alone a brand new idea) but you will find the devil is in the details. What makes or breaks those sites are the thousands of minute implementation level decisions which build up to make a quality application.
Complex software takes good knowledge of complex algorithms and man-years of development. Well.. since everybody wants to get rich quickly these days, innovation scaled down to web "applications", where the only valuable thing is percisely the idea. And your only advantage is the time to market.
It took years for other big firms to develop search engines as good as google's is. THAT is what I call "implementation is hard". Implementing something as trivial as myspace is less complex than compiler class project in college. (I've done both).
Why do hundreds of kids bother applying to YCombinator?
Can't they just whip up a webapp in a few months?
Yes - web dev is easier than desktop app or complex client/server applications, but Hitting the High Notes (as Joel says) will never be "easy". Simple, yes. Easy, no.
Hate to sound negative though, having that happen to you surely hurts...
And yes, kids apply to YCombinator exactly because they can whip up a web app in a few months. YCombinator puts them in front of VCs after that.
There are always exceptions to every rule though, re: the original point.
i.e. MySpace. Talk about crappy technology (or so it would seem; they've managed to scale well), and a has-run idea.
These are webapps, not operating systems, not mainframes, not databases. The barrier to entry, cost of operation, and development time are all pretty low. You can hire the PR people when you actually need them (probably sometime after you actually have cash flow). There's plenty of ways to market a webapp for free, and I sure would not hire a marketing firm to promote my new site.