Metro 2033 practically drips with delicious atmosphere. It features excellent visuals: DirectX 11 allows eye-popping animations of which few games take advantage. Metro 2033 has beautiful lighting, shadows, and tessellation, and explores motion blur in good ways.
The flaw, in my eyes, is that you are asked to make choices early in the game that could make the game un-finishable later. In particular: you can only purchase good, full air filters for your gas mask at the beginning of the game. This means that a standard strategy of conserving your cash at the beginning until you know what you really need will result in a failed game. You'll get through 2/3 of the game and find that you can't continue, because you can't scavenge enough air filters on the way. And because it's a game-on-rails, you can't go back to the beginning, or take a side trip to a marketplace. You're stuck.
This style of gameplay, this artificial difficulty, is a tactic of early adventure games. Asking you to restart the game and try again smacks of having your lantern stolen by the thief in Zork I. You're hosed, but you don't know it yet. And when you find out, you have to restart, and waste all of the effort you've made.
Games today generally let you make bad decisions, but they also let you recover from those decisions. This is different than games being "hard" or not hard" - I'm talking about coupling two different game design mechanics that don't go together well: 1) linear progression and 2) being able to make choices, in a knowledge vacuum, that could break the game.
After realizing I had broken my chances of finishing the game, I looked around online and found that this is a common complaint of Metro 2033. But, I also learned that the game has several ways to get different endings that require you to do certain strange things in the beginning. For example, to collect particular items from different locations and bring them together. This means you'd have to play the game several times to get these different outcomes. This confirmed for me that the "gotta know to buy air filters" problem was probably a design choice, an intentional punishment. BOO.
Of course, Metro 2033 is frequently on sale, and I've seen it in a Humble Bundle, so it's hard to complain. If you buy it, just remember to stock up on air filters at the beginning!