Gaining extra life energy, machine gun upgrades or secondary ammo while the respective item is full/fully powered gains more points for doing so. This encourages not getting hit, not using your rockets or whatever willy nilly and, in the case of the machine gun upgrades, not dying because the machine gun upgrades disappear with death. Better play skill is required to avoid death, avoid spamming your secondary weapon and it’s even harder, of course, to completely avoid being hit.
Also, the more you can kill without being killed will usually reward you with a Damage Streak, which will amount to much greater points/experience. The increase in points/experience that you get from chaining together kills without dying in turn allows the player to gain more levels in a stage and will let them get perks earlier. A meter appears when you have a Damage Streak going and becomes a visual representation of how well you are doing in the destroy everything that’s in your way whether it be foe, explosive crate or turret department. The meter on its own urges the player to keep moving, keep killing and to play as best as he can to continue the mayhem.
This is a feature that I’m sure many hardcore players would love in more games, and even the most average player will get these bonuses from time to time making them feel better about their level of play as well. It’s a win-win situation on all levels.
You get rewarded for playing like a maniac, aka having fun: Driving off a cliff garners stunt points. Driving through a building garners destruction points. Blowing up miscellaneous cars, barrels and the like get you the same. Combined with the fact that the game doesn’t penalize you for ramming head on into things, and here you have a game that wants you to have fun. This is yet another simple idea, but one that seems like it is easy to ignore or just forget that the first and foremost goal should be facilitating fun and not just punishing the player for being reckless.
It should also be noted that what I was mentioning before about skill applies here too, because pulling off a crazy jump off a ramp while shooting a rocket to blow up a barracks behind you while falling into a gun turret tower and machine gunning everything around you will amount to huge damage multipliers and as such just further rewards the player.