Bowdlerise: The German version renamed the Nazis to "fascists", due to Germany's heavy laws about media dealing with the Third Reich. It's more subtle than it seems — Russians do often refer to Nazis as just "fascists".
Plot-wise it's also reminiscent of Hellgate: London, which had pretty much the same premise, only with demons instead of mutants.
Keep Circulating the Tapes: The original version of the game was removed from digital distribution upon the release of Metro 2033 Redux, even though Redux was essentially a complete remake of the game rather than a remaster. This means anyone wanting to experience the original Metro 2033 will have to find a Steam key or get the game on Xbox 360, and the Steam version of the Ranger DLC is completely lost.
The Other Darrin: Artyom's voice actor was replaced in Metro: Last Light, and said actor also re-dubbed all of Artyom's chapter intro dialogue in the Redux version. In addition, several voice actors were also replaced in Redux.