[freeflight]

From the tech side it was the heavy, and smart, use of scripting that made HL1 stand out.

With HL2 it was the added physics engine trough the change to Source, back then that used to be a big deal and whole gameplay mechanics revolve around that (gravity gun).

In that context, I do not really consider it that surprising for the next HL project to focus on VR because even early demos of that combination looked already very promising 5 years ago [0].

[0] https://youtu.be/-RehCTRrWM0

[glenneroo]

Here's a nice run-down from Valve News Network after he/they(?) dug through every patch for all of Valve's games in search of anything related to HL3 and VR: https://www.youtube.com/watch?v=VaXGA_wSWDA