I'm still a svcker for Half-Life. Not only was the first one an amazing step up in FPS storytelling when it was released (and therefore I have really fond memories of its release), but I still enjoy the clean lines, polish, and succinct simplicity of Valve's (both graphical and narrative) design style. I'm sure that a lot of the members here weren't PC gamers in 1998, and so don't share in my nostalgia.
I'm still looking forward to HL2 Episode 3. No, there isn't going to be a Half-Life 3 anytime soon. We're still waiting on Episode 3 of HL2.
That said, as has been mentioned, E3 is such a huge media feeding frenzy nowadays it almost doesn't make sense for a company like Valve to bother with it. They just came off of a huge, multi-faceted marketing campaign for Portal 2. They don't really need that kind of watered-down exposure right now, IMO.