Scout: You run fast. That's about it.
And everyone else can't? That sounds about right
Stalker: You can turn invisible (more like translucent, what's the point of being invisible if people can still see you? this blows my mind)
You aren't supposed to be completely and totally invisible because that would be incredibly cheap.
Airborne: You have a jetpack. Clearly people are going to use this class the most because the others are useless.
From what I hear, the jetpack is mainly good for getting up to advantageous spots quicker. Not to make like Jango Fett and rain death from the skies.
Guard: You get this ability where you pound your fist into the ground (doing no damage) and you cannot move, but you are surrounded by a strong blue shield. During this time, enemies can go ahead and stop shooting at you while you are trapped trying to plan out your next move. After the effect expires, a small shockwave is emitted from you, and you regain control. The entire effect lasts about 5 seconds, in which enemies can stop firing at you, back up, wait until you're vulnerable and then shoot you some more.
*cough* *cough*Maybe you suck at the game and try to use it while fighting one-on-one in which case it would be useless.