Bungie managed to create a game with an eerie atmosphere, rich story and enough lore to keep you interested. Marathon is more of a story-driven puzzle game with fps elements than a shooter and you should treat it as such when going in. The game is about 8 hours long first playthrough which is the normal length of a Bungie game I've used to.

The two AI characters, Leela and Durandal, that the game is centered around are very well written. Durandal has to be one of the best little shit characters in gaming ever. The other characters are mostly enemies that don't talk but have personalities based on their gameplay mechanics. Sometimes you encounter friendly human units but they are only a big nuisance because you can't run through them as they have collision.

The level design is not bad as some people say. I think it's actually very good. You're always on your toes with enemies around the corner and the puzzles keep your brain busy. There's not a single boring moment in the game. Sometimes the save points can be sparse but the levels themselves are so short it isn't an issue until later in the game. Lack of jump is no issue at all because the levels are not designed to support jumping.