Source engine is NOT liminal. Half-Life 2 is. 

yeah, you heard me right. Source engine does not carry any liminality to it by itself and i saw this blatant misconception going around for quite a while. 

Source is... an engine. without assets and game logic it can not create any mood or artistic intent. and very often the very first game that was released using a primairly in-house engine like Source, more often than not it sets the artstyle and athmospheric DNA for next games to come. First ever game released that was based on Source, officially, was Half-Life 2. and I'll circle back on why this fact is important in a bit.

when talking about liminality, have you noticed what games are usually brought up?

• Garry's Mod
• Half-Life 2 itself and its derivatives like HL2:DM
• Counter-Strike: Source
• Portal

but... why not bring up Portal 2? or Left 4 Dead? or Team Fortress 2? well. because they act as a counterpoint to the whole "Source is liminal" schtick.

Portal 2 doesn't feel liminal - it's a satire about ego and hubris. Left 4 Dead dilogy doesn't feel like that - even when playing solo streets are full of infected and you immediately know it's a zombie apocalypse - cause and effect, you know. the only exception is playing TF2 solo, but that's an unintended side effect of any usually bustling and active multiplayer game being empty, that general feel of wrongness.

so... what is common about the games that are used as a proof? what makes them actually liminal? and here's my observation about Half-Life 2 being the first official Source game comes back.

all of these games use Half-Life 2 assets as it's base asset pool. they essentially inherit Half-Life 2's art direction and athmosphere. some partially (like CS Source), some wholesale (Portal, GMod). Portal 1 Aperture soundscapes are literally slightly modified Combine Citadel soundscapes. and GMod just inherits the entirety of HL2 asset pool wholesale, and a lot of maps are made using it. 

Source is an engine. by default, it has no athmosphere, no mood, no design language. but these games are based on DNA that Half-Life 2 set for the first decade of Source. Half-Life 2 is liminal - and by design.

Half-Life 2's main theme is post-apocalypse. and a rather different kind of such than L4D. it's an opressive post-apocalypse. Earth is defeated, devastated, and mercilessly milked for resources by Combine, an intergalactic superarmy, moving through space like a force of nature. Half-Life 2's art direction is strictly designed to infuse the sense of wrongness as you cruise through empty streets, ride an airboat through canals or get through coastal wastes on a buggy. you are supposed to feel small, insignificant even, to the grand scheme of how the world is now. its the melancholy and liminality of an opressed world. opressed by a force orders of magnitude more powerful than this world ever was. now withered, abandoned, with only a few flocks remaining of what was humaninty.

and when you load into an empty roleplay map directly based on this very asset pool, especially with soundscapes, or wander around old design Aperture Science, hearing distant and indifferent humming of the Citadel, you get the same feeling. the art direction that was inherited from Half-Life 2 is doing its job. it gives you this sense of liminal loneliness in a world long gone. something being stuck between life and oblivion.

and this is why Half-Life 2 is liminal and passes that DNA on other early Source games. Source itself does not have anything to do with it.