I've said this before, but the biggest takeaway from the balsamiq story seems to be: put your head down and focus on making a great product, everything else will follow.
And market the hell out of it.
The Balsamiq approach to marketing (market the success story) is quite similar to that of 37Signals, actually, and both essential and very effective.
The "build a great product and they will come" myth is bullshit 99% of the time (unless you have a built in marketing engine-- SEO or viral). Balsamiq won the same reason that 37s did (and JoelonSoftware for that matter)... They had a great story, told it well, and the story happened to resonate/be interesting to their exact target market (web geeks / entrepreneurs).
Bravo!
But try to build great software for supply chain management, or managing a beauty parlor and let me know how "everything else will follow" there. There's a reason that SalesForce.com (which, arguably, has/had a great product) has spent 60-70% of their topline on sales and marketing.
[1] http://codesynthesis.com/~boris/blog/2008/11/10/develop-new-...
1. Make something people
2. Lots of hard work
But, YC is almost always investing in companies that have yet to make anything. Their entire business plan is about trying to predict which founders will have these two qualities.