Okay maybe its not the most well-thought out theory, but here it goes anyway. I think the real reason HL2 is being delayed is because of the graphics. Don't get me wrong--they're great but they are not revolutionary, they have the potential to be though. When compared to Stalker and D3, HL2 falls a bit short of next-gen graphics. It is relying on high-res textures and eerily realistic character face expressions to do most of the work, the lighting is good too, but not great (especially on other character models).
Valve has alot of expectations put on them, more so than D3 and Stalker (of course, time will tell which is the better game). Nowadays, graphics are one of the primary factors by which a game gets judged. So, I think Valve is trying to improve the graphics, like the lighting and whatnot as well as tweaking the physics engine. I don't care about graphics really, HL2's are more than sufficient, and it is the #1 game on my list, but as many others have pointed out (and its fairly easy to see why) Valve's entire reputation is at stake with this game. At the time of HL1, it had state-of-the-art graphics. I think when D3 and Stalker appeared on the scene, Valve reasoned to delay the game while tweaking the engine for graphics and optimization. Thats just my opinion though.
Valve has alot of expectations put on them, more so than D3 and Stalker (of course, time will tell which is the better game). Nowadays, graphics are one of the primary factors by which a game gets judged. So, I think Valve is trying to improve the graphics, like the lighting and whatnot as well as tweaking the physics engine. I don't care about graphics really, HL2's are more than sufficient, and it is the #1 game on my list, but as many others have pointed out (and its fairly easy to see why) Valve's entire reputation is at stake with this game. At the time of HL1, it had state-of-the-art graphics. I think when D3 and Stalker appeared on the scene, Valve reasoned to delay the game while tweaking the engine for graphics and optimization. Thats just my opinion though.