Originally, the game was going to be called Serious Sam: I Hate Running Backwards, however, they've ended up adding in a lot more content than they originally planned. It still has Serious Sam as a playable character, but there's other characters in it now too.
It will have non-stop action, daily challenges, local co-op multiplayer, randomly generated levels, a destructible environment along with vehicles. It looks absolutely insane, take a look at a beta trailer from late last year:
In terms of vehicles, they've said there will be cars, boats, mine carts and it will also have a monster so big you need to jump into a helicopter to fight it—sound glorious.
It's due out sometime around March/April this year.