Dead aliens leave behind Data Points which you can collect to upgrade your equipment.
At the start of the game, not many weapons and power-ups are available.
Finding question marks makes new objects available for research between levels.
With a name like "Nuker", you know this weapon is going to do some damage.
In addition to xenocide, your missions will occasionally require that you find and rescue trapped humans.
The spinning VibroBlade XL will turn you into a lawnmower and the sectoids into grass.
Collect the letters to spell BONUS on a level and when it ends, you'll get to play a quick bonus level (imagine that).
Most bonus levels are mad dashes for extra Data Points (which can be the difference between getting a good upgrade or a great one).
Ultimately, there are a dozen weapons, a dozen power ups and half a dozen enhancements available, each with four strength levels.
At the highest upgrade level, the grenade launcher spits out an insane number of explosives.
Winning the game unlocks a set of bonus skins - like the Elvisforcer.
Hmmm... why does this bonus level look familiar?