Ten signs you might be playing an Old School Shooter:
#10. You often shoot allies because their texture resolution is almost indistinguishable from the bad guys
#9. Assuming you find an ally, they are usually already dying and live long enough to impart a few words of wisdom or direction to you before messily expiring. Bonus points if an alien pops out of them.
#8. Everyone builds fabulously expensive complexes filled with steam pipes that serve no discernible purpose other than to act as a launch point for an invasion (to or from, doesn't matter).
#7. Said complexes often contain numerous rooms that are clearly designed only to serve as murder holes and ambush points.
#6. Everyone...absolutely without exception...wants to kill you. There are never any janitors, engineers, or passers-by who aren't either interested in your death or about to turn into an alien and then kill you.
#5. You are more than halfway through the game and either have no idea precisely what the plot is or why you're where you are at.
#4. You're more than halfway through the game and have realized that there is no plot.
#3. Your life depends heavily on random canisters chunks of meat, or convenient medi-kits lying around randomly.
#2. You never, ever carry enough ammo to take on a proper army, even though you are, and random ammo stashes are conveniently everywhere, no matter how unlikely or alien a world you are on.
#1. You regularly meet very large, tough opponents who can only be harmed by hitting a glowing six centimeter wide spot where they accidentally exposed their pulsing cyber-heart.