The biggest draw about Up your Arsenal was the multiplayer. Most of the modes of it were similar to most shooters, but one mode sets it apart from ANY SHOOTER OUT THERE: Siege Mode. It's basically where two teams of up to 8 each defend a base while trying to take the opponent's base. There are no reprecussions for dying in this mode scorewise. You just start from square one with your weapons.
The story is what you'd expect from a Ratchet and Clank game, but there are some definite changes, including a refined health meter with HP, rather than those Nano-Bar things from the first two.
Weapons also level up alot faster, making it easier to upgrade things later in the game. Also, there's two refillable healing items in the game. It helps for the later areas, believe me.