That the games have absolutely zero cuts and loadings.
No loading screens, I'm pretty sure. I haven't played through the entire games though so I'm not sure if they have absolutely no camera cuts, but I do know that the cutscenes are within the gameplay itself and don't cut to a different camera.
EDIT: I guess I'm wrong, there are apparently loading screens.
The loading screens are very brief, and the game doesn't really rely on them (for anything other than getting around technical limitations).
A remastered/remade Half-Life 2 could be made without loading screens, without changing any game content.
Meaning the game was limited by the tech at the time (2004, Source).
There are no cut-scenes in Half-Life 2.
The closest the game gets to having cut-scenes is with:
Basically rendering a portion of the world outside of the player's sight (or even inside it), and piping it into another part of the world.
This is used to handle the rendering of characters/scene through monitors.
This is first seen in Point Insertion: where the player sees Dr. Wallace Breen, giving one of his Breencasts in City 17.
Half-Life 2 also has scripted scenes to tell the game's narrative, however the player is generally able to run around whist the story is happening.
Last edited by caffeinade - on 27 March 2018
Half-Life (1) is slightly different.
There is a section (roughly in the middle) where there is a cut.
Portal and Portal 2 both have a cut-scenes, and both have loading screens (built on the same engine as HL2).
In short: Half-Life and Portal are great, but were limited by the technologies of the time.