While Metro: Last Light has taken the post-apocalyptic franchise to new heights, a lot of gamers (especially those on PS3) missed out on the original Metro 2033. It's got more bugs and some serious design changes than Last Light, but it also offers are far more genuinely harrowing experience for 360 and PC gamers. Even on Normal, there are some scenarios that reach Silent Hill level of "oh my gosh how am I going to survive this?"
Stealth options are more problematic so those who played through Last Light first on a stealth run will have problems here depending on whether you used silenced weapons or knives. Despite those issues, the game feels far more visceral and far less cleanly handled like Last Light. It feels genuinely like you are clawing for every inch of life, so those wanting a more authentic, less shooter-y experience should definitely try the original.