I think the upped enemy variety is amazing even today and the fast paced almost John Wick sometimes feeling combat is still stupid slick after all these years.

Where it shows the most cracks these days is maybe it's level design and boss encounters. The Icon of Sin fucking sucks and there's an almost (albeit really charming and personality driven) amateurish level design that can sometimes make you feel stupid for not seeing the tiny crack in the wall that lets you progress.

Other than that, it's still Doom 2 and it still fucking rules. The mod scene for this game will never not be amazing too.