With a huge city to explore and the new Firefight mode, ODST may be short, but it's highly replayable.
You take on the role of an ODST who is only referred to as "The Rookie." He is the FNG of a squad of ODSTs. Initially, you are set to infiltrate the Prophet of Regret's carrier ship, but O.N.I. has had a change of plans and has reassigned you and your squad. However, you do not know this yet. Six hours after the drop, you are alone, still in your drop pod. You need to find your fellow ODSTs and figure out what had happened during the time you were out while mysteries unfold and you figure out the mission O.N.I. has given you.
Halo 3 ODST told an amazing story with a great atmosphere that spelled the word "alone." The soundtrack further emphasizes this with great ambient jazz music. Covenant patrol the city and you need to be on high alert. Snipers are out to kill you when you least expect it and Covenant patrols are constantly roaming the city.
I would also like to mention that the city is huge for a game that surprisingly isn't considered an open world. The gameplay is mostly the same as Halo 3, but what would you expect from an expansion? Gameplay is very different during the Mombasa Streets levels and strike a new feeling in Halo: That you are alone.
You switch between the huge Mombasa Streets and flashback to your other ODST squadmates. Think of Mombasa Streets as a hub for all of Halo 3 ODST's levels.
Even after playing through the short campaign, the size of Mombasa Streets makes it great for exploration. The game's lifetime is extended by exploring the city and making it a never ending city of Covenant patrols for you to fight and it's extremely fun. Think of Mombasa Streets as an open world game.
Beyond campaign, we have Firefight, a game mode similar to Gears of War 2's Horde mode. Here, you progress wave after wave of increasing difficulty. This is a great addition that extends the game even further.
Halo 3 ODST is my favorite Halo game. Despite all the negative response it has from the fans, it really isn't that bad. The atmosphere was great and I could feel the mystery in the environment that was futuristic, yet strangely human.
I really felt like I had stepped into the shoes of each characters. The voice acting was excellent. The atmosphere was spectacular and had a lonely noire type feel. Soundtrack was the best there was. Mombasa Streets was like a lonely open world for you to explore. Each member in the team had a charm to them, unlike the cold Noble Team, from Halo Reach, who were almost completely flat.
All in all, ODST is under appreciated by the internet.