David left the below in the comments, but I'm bringing it out front, because it's a great clarification and enhancement to what I had written:
1) The Most Important Person on the Startup Team
In a blog post that I wrote on the subject after it came up in a conversation on the nextNY mailing list, I posit that it's not "the techie", nor "the UI person" nor even "the biz guy"...but rather The Entrepreneur... someone with a special set of skils and characteristics that may—or may not—be co-resident with the other functional skills mentioned above.
2) The Ten Crucial Attributes of an Entrepreneur
Although I haven't yet taken the time to blog about this one yet, it has been a staple of my business school lectures (and was captured by the New York Times during my Ignite presentation last month). In a nutshell, I have found that most investors look for the following ten 'must-haves' (pretty much in this order) in their search for the Perfect Entrepreneur: Integrity, Passion, Startup Experience, Domain Expertise, Functional Skills, Leadership, Commitment, Vision, Realism and 'Coachability'.
3) The Entrepreneur/Investor Disconnect on Returns
My point here was that even if you could get a typical entrepreneur and a typical investor to agree on the same target investment return for the investor (say, 25% IRR, as a reasonably high return for investing in a really risky startup), there is a gaping chasm between the two, because the entrepreneur looks at the question in light of his or her own venture, whereas the investor looks at it in light of his or her entire portfolio. The result is that the entrepreneur has heart attack when, having come to such an agreement, the investor says, "great, now that, of course, means that I need to get thirty times my money back from YOUR company! I've gone through the math in detail on my blog, but a crib note version is available over on Center Networks from a presentation I gave last Spring.