Steve Krug — American Businessman

Steve Krug is an information architect and user experience professional based in Chestnut Hill, Massachusetts. He is best known for his book Don't Make Me Think about human-computer interaction and web usability. He also heads a one-man consulting firm called Advanced Common Sense. Krug offers in-house consulting sessions where he lectures on usability testing and provides targeted advice to clients on web usability strategies. Krug published Rocket Surgery Made Easy: The Do-It-Yourself Guide to Finding and Fixing Usability Problems in 2009... (wikipedia)

I don't think there are as many usability issues as there are tactical or strategic decisions related to whether incorporating social networking into your site is going to help or hurt.
When you go to a site, you usually run into usability problems pretty quickly. They're not hidden. They're not complicated. They're not baffling. They were in the design or crept into the design.
If you're going to implement video, do it as well as YouTube does it or don't do it at all.
The general reactions were that the video was either not going to load, or be painfully slow to load, or would require a plug-in users didn't have. YouTube changed that, because it just works.