Humans are not especially graceful creatures compared to our four-legged rivals. This trait is never with one exception is never properly simulated in games which results in players being able to turn on a dime when running full speed. That one exception being the Gears of War series where sprinting reduces the turning speed of the player significantly. I would like NS2 to become another exception.
As a contrast to the superior four-legged mobility of the aliens (in particular, the skulk), I would like the Marine sprinting ability (normal walking/running is unaffected) to be sort of clumsy. When sprinting, the marine lowers his weapon, cannot strafe and increases his movement speed as in any other game. However, in addition sprint will bestow these additional properties:
1. While sprinting, the player cannot rapidly change direction without losing significant speed. The marine needs to make smooth turns to maintain a fast speed. This is implemented by there being cumulative speed penalty for moving the mouse on the horizontal plane (as in moving the mouse to change direction of movement).
2. When ending the sprint, momentum is kept and the marine will need to decelerate for a couple of seconds (shorter if the marine was moving slower). This means the marine cannot stop at the drop of a dime and runs the risk into walls into walls or falling off ledges if not careful.
It's sort of a weird idea but I think it was way of adding depth and immersion to the game. It also introduces mechanics that would be normally seen in racing games or games that have their protagonist ride on horseback (most notable, shadow of the colossus). A marine to fully take advantage of the ability to run away from certain death, needs to be able take the most optimal route in a situation while avoiding obstacles (like skulks and other marines).