I really like the way the this letter is crafted. It's sensitive to the founders, and is upfront about the reality: there are more promising groups than available spots. I'm struck not only by YC's admission that they will invariably miss good groups, but that they'd like to keep in touch to learn from their mistakes.

You certainly don't see that humility everyday from institutions of any size. Other organizations should take note. A personal tone goes a long way.