Early trailers suggested a story based, Real Is Brown FPS with RPG elements. It was changed to a colorful, Rule of Fun FPS/RPG with focus on More Dakka. There was an entire story that was mostly cut out, which actually led to one of the devs leaving the game industry forever.
The fans and critics alike were disappointed by a lack of storytelling and the Final Boss being a Giant Space Flea from Nowhere, which most likely caused the developers to put a bit more emphasis on story in the sequel, though it still retains the run, shoot, and loot gameplay from the first game.
In the Game Informer article, they mentioned being able to disassemble guns into their parts so you could build your own super guns out of them. Not that it matters when you can find almost any combination you could want in game anyways (or build your own from component parts outside of the game with a gear construction kit).