The Plenary Day was not that interesting this year. There was a Developer Day the next day, which was probably better, but I could not make it due to a conflict with the HTML WG meeting (labeled HTML5 WG by the W3C Team for unstated reasons). There was the recurring HTML5 and extensibility panel. Organized each year since 2007 to gather some thoughts on how to solve a problem of which it is not very clear what the problem actually is. The whole debate made me think about the Hitchhiker's Guide. The answer is XML namespaces, but what is the question? Admittedly the comparison with unicorns and ponies is a lot cuter and likely more accurate. Unicorns in particular dominated the entire TPAC "secret" backchannel (#unicorns on irc.w3.org:80). Actually, not just the backchannel, they appeared in Working Group meetings, questions, presentations, and editors are adding them to specifications. Lovely.
The problem with the Plenary Day is that there are three-hundred or so people in the room and at least half of them is not paying attention to what is happening. Running the whole thing in unconference-style (maybe even TPAC entirely as Hixie suggested) would probably be much more productive for most people. So while some people are debating WS-DeathStar in one room I could be having a great time discussing the intricacies of the CSSOM with just the people that care about it without having to bore anyone else or having to explain all the background information to the one person that was not prepared. I am not really persuaded that this would go against broadening your horizon because I would not really be following a discussion on WS-DeathStar anyway.
TPAC is supposed to be a place where groups with some overlap in interest can work together, but in practice it is only a subset of the groups that has a particular shared interest. Given that I think it is much more productive if only the subsets meet (and maybe report back) rather than having an additional thirty people in the room who could not care less and essentially just take up space that prevents the interested people from sitting closer together and communicating more directly. The uninterested can also take up a lot of time in the round for introductions. Furthermore, the uninterested could be the interested in a different room with a different topic.
Another thought I have had about TPAC for a while is that it might make sense to have such a meeting for all Working Groups of which the deliverables essentially evolve around Web browsers and Web authors. I realize not everyone is a fan of that characterization but when it comes down to it there are a few groups where implementations in Web browsers that can be used by Web authors are the primary goal. I.e. WebApps, HTML, CSS, Geolocation, SVG, and a few others. Having at least one event per year to coordinate on the direction of the Web Platform would be really good I think. (Come to think of it, also a good target for terrorists.)