The signup pages are an example of a broader mindset which is different at Tumblr and at Posterous and which in turn, to me, exemplifies broader trends for startups. That's what I think is interesting.

I'm not saying startups should not be metrics-driven. Design and engineering are both driven by metrics. But they're different mindsets. And in this case, one is clearly winning.

You're also right that the zero-sum fallacy is wrong. However, those two companies are competitors in the market and one, at least on publicly available data, is winning hands down.