Some games seem bad because they are. With Metro: Last Light, it's more a question of expectations: what do you want from the Metro series? For me, it's a scary and dark post-nuclear Russian underground, a first-person survival horror-slash-shooter with scarce resources and terrifying scenarios. There's a bit of that, to be sure. But if you also want tits, QTEs and hand-holding companions, then congratulations - you're part of the wider audience this game is looking for.
Metro: Last Light is a game torn between the niche its predecessor staked out and genre conventions that are perceived to help sales. So, to give one example, you're accompanied for large stretches of the game's first half - a crutch at the best of times, and one that rather taints the atmosphere here. Not that your companions Anna and Pavel are dislikeable characters - the latter's a chatty rogue with charm and surprises to spare - but their presence feels unnecessary.
Metro is based on Dmitri Glukhovsky's series of books, and the pitiless dystopia crafted over two volumes is one where humanity bleeds, both figuratively and literally. Metro 2033 caught this atmosphere in its tight corridors, and Last Light does the same, albeit with more caveats.