-Every two-handed weapon, except for the Shotgun and Grenade Launcher, has a Precision Mode, be it a scope, a shouldering of the stock, or a laser sight on the Alt-Fire key.
Using any of those gives you more precise aiming and tighter spread, at the cost of slower movement, and the disabling of your jumping, running and rolling while active.

-The Shotgun and Grenade Launcher have alternate ammo, instead.

Shotgun gets Wideshot, which increases damage and horizontal spread, and the Grenade Launcher gets Super Grenades, which turn your grenades into room-clearing cluster-bombs. Toggled with Alt-Fire.

-Dual-wielded guns can be single-fired, halving ammo consumption and damage, but increasing your rate-of-fire.
Really fun with the Uzis. Press Alt-Fire to do just that.

-Crouch + Jump and Directional commands roll, on ground and on-air.
While rolling, Lara takes a fraction of the damage she receives from most attacks.
Rolling can boost you forward in mid-air, and could (hypothetically) be combined with a sprint jump.

-Lara moves slightly slower than the average player, but she can sprint with the Run key.
Sprinting makes her run at twice the speed, and lets her jump farther.
Lara can't sprint while using Precision Modes, and her stamina regenerates when on-ground and not sprinting.

-FRG also has a "holster weapons" button, which lets Lara climb ledges.
Pressing Use climbs them, and the maximum height she can climb is of the average Doom crate.