Product, product, product
I found this Dropbox presentation absolutely charming, not only because I am an avid user of the product, and a passionate fan of the product, but because it speaks so honestly about something that should be front and center for every single startup that uses the Internet to deliver its offering: your product matters— in fact it matters almost more than anything else.
As we fall in love with all sorts of new concepts around distribution: virality, social game dynamics, flash sales, etc., it is critical to remember that these are tactics to expose new potential users to your product (or service), and that no matter how well they work, at some point you will go splat if the product is not unique enough to delight some subset of your potential customer base.
Product, product, product. Say it 10 times and don't forget it. If you are a startup, this is the only way that you will build deep and sustainable value. While investors (like me) may ask you questions about distribution, these should only be coming after they already believe you've cleared the first threshold of having something people will actually want.
[And to those who might point to the failure of the Betamax in the 1980s or the Mac in the 1990s, I have good arguments for why these were both bad products for the markets where they lost. As is probably the case for many other counterexamples of great products losing to sales & marketing]
I'm a VC at Matrix Partners living in the Boston area. I've started some stuff, worked at some
places, and I love making things.