File this away next to "A good product is 95% of marketing" and other patently untrue things engineers like to believe.
Quality content and linkable content, for example, are not coextensive sets. You could write the world's best guide ever to cross-stitching school uniforms (did I just make that up?) and you'd get less links than DHH gets for not cursing during a Rails keynote. If you're doing SEO and you haven't figured out that linking behavior is very different in different audiences, I'm as worried for your future as I would be for a salesman who was doing high-touch enterprise software sales for rooms full of third graders.
There are also easy ways to shoot your great content foot off, and I really wish I could pull in examples from clients here. Hypothetical example: suppose a YC-style company is founded by a noted industry expert who has a great personal brand. They produce a ten-page guide to a particular new technology their startup uses. The guide is, far and away, the best quickstart guide on the Internet with regards to that technology.
Q: Their startup benefits from this a) a lot, b) a little, c) virtually not at all?
A: I don't know. Where did you post it?
Q2: Come again?
A: Like, physically, on which page does that best-in-class guide exist?
Q3: Oh, our Posterous / Github account / etc.
If it markets itself as bug-ridden excrement piled on refuse- Well then! you're hosed next time your customer goes looking for a solution.
More succinctly: can a poor product succeed with good marketing? For the long term?
Perhaps, but I think 'patio11's point was that even if the product is great, it could be worthless without the right marketing/promotion/SEO/etc.
So you could be anywhere from 0% to 100% of the way there with just a great product.