The Russians have done it again, superb ambient experience albeit a bit rough on the edges.
What IS special about Metro though is the story and all the obvious effort put into the screenplay and directing. Character development was in my view comparable to such greats as Assassin's Creed or even Half Life 2 (I bet at least 50% of all you hl2'ers out there have had a crush for Alex at some point ;)). The supporting characters were very believable, their struggle for the common good and facing the hardships of life in the Metro seemed natural and was a great help in creating a truly immersive atmosphere.
So there's nothing concretely negative I could point out about the game. Maybe one could complain of the lack of multiplayer game-modes or that the replay value isn't as good as some other games of the vast FPS genre. Still, this game, as was Stalker, is in my view made in the form of a work of art - it is designed to be enjoyed from beginning to end as a whole and both multiplayer and replayability would seriously dampen that feeling. It's just the same with movies - some movies you just want to watch once while others can be enjoyed countlessly with friends!
In conclusion, while the game falls just short of being truly epic, as a whole it still is a great game well worth a try especially if you're into post-apocalyptic shooters. One can only hope Metro 2034 isn't far away on 4A games's list, provided the same people direct and write the story to it!