Anyone read the recent interviews with Gabe Newell? He spoke about a Half-Life movie somewhat. They tried writing a script for it, some Hollywood guys tried too, but at the end, Valve thought it wouldn't make a good enough movie. Well, see for yourself what he said.
Anyway, Half-Life compared to Half-Life 2...
I think I liked the first most because it was refreshing, different than most other shooters. Half-Life 2 isn't a bad game, it plays away nicely and there's a lot of fun to it, but I felt that special thing was lost a bit. Maybe that's always the case with sequels, but maybe it could've been better than this.
From a mappers perspective, I liked Half-Life's diversity in enemies and weapons much more. I like Half-Life 2's new technology and increased possibilities though that makes production time longer and it's art content has quite some impact on mapping as it was. Nowadays anyone can make a map look good because of it's textures. Back then, those textures weren't so good and you had to create actual good architecture and make the map flow well. That last part is still the case, luckily...
Anyway, I think a lot of people are pulled out of it now that it takes a texture artist and a modeller more to make a map stand out. It's not just mapping anymore. HL2's content gets quickly overused and only fits the styles and themes it was made for, it gives little freedom for other themes I think.
My 2 cents.