I'm willing to guess that GMail is among the top 3 most used products by HN's readership. This is a substantial change in the experience of that product, as well as deployment of what seems to be some pretty nontrivial technology powering that. It's also the kind of feature that will impact any startup that uses email as a communication channel with its customers (to wit: All of them).

If that's not relevant to the HN community I'm not sure what is.