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.
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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.
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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.