Skimming over the D3 source, it all looks so very readable. Which is impressive in itself.

A bit off-topic, but I'm curious: why do you think that's impressive? I thought it was pretty commonly accepted these days that readable code saves more time than it costs over the life of a project...?

Because it may be commonly accepted, but it's rarely actually done so well.

Game code specifically is usually the exception to this rule, because it tends to be written to very strict deadlines, ad-hoc requirements, short project lifetime (no need to maintain or support it for years after), and not much expectation of re-use.

This does not all hold for Doom3 as it was also an engine sold to other companies for other games.

A lot of things are generally accepted as "good ideas", but how often are they practiced? Most software management would throw their own mother under a bus if it got the product out the door on anything close to schedule or budget.

Which is to say, code cleanliness is very much not the norm anywhere. And if anything the game industry seems to be a good 10-20 years behind the rest of the software industry in terms of hewing to good practices (clean code, automated tests, etc.)