Ehhh, you are right to some degree. No, I don't think that they revealed every feature of WP 8.1 yesterday, but that did list the major selling points. Im sure there are a few hundred minor tweaks that make the general experience better, but all I got out of yesterday was Cortana, messaging center, any-size tiles, and transparent tiles. If there was something else to really get excited about, they would have mentioned it. Most of that stuff we already knew about, and really, who do you think even leaks that info? Its probably MS and Nokia. If they had something new and exciting to talk about, they would have done it yesterday, if for no other reason than that they knew that most people were aware of the new features. It would have made sense to say "see we have all those features, plus a whole lot more!". Instead they said "yeah, that's about it".
While I know that this is a developer conference, new features and phones are VERY important to developers. They need a reason to be excited about the OS. They need to feel like these things are going to sell so that their apps will sell too. What other high-profile Microsoft events are happening in the near future? Does it really make more sense to make major announcements when only the people who care about WP right now are going to hear about it?
They goofed. I know a lot of you cant accept it, but they did.