Yahoo's Jeff Bonaforte really has stirred bloggers with his video dissing of Google usability. But it shouldn't surprise anyone that Yahoo is making the conversation about usability, given efforts like their Design Patterns Library and open source Web 2.0 UI Library.
While I'm curious to know how many takers their open UI has had, it's a smart move for Yahoo to promote their products as the most usable choice. Commodities like Web-based email have all the features people need, so to compete in these mature spaces, usability becomes an excellent way to differentiate products.
However, it's disingenuous for Yahoo to credit Google with success only at delivering search results, when their breakthrough really was making search hugely profitable with AdWords. AdWords is a very usable, user-friendly way to deliver ads.