Londo-X said:
...or maybe modern millitary with each branch of the service being it's own class...
While this is a common consideration, I can tell you from personal experience that the branches in and of themselves are as varied as the branches compared against eachother. If you plan to do that, you may want to consider making the branches the equivalents of races. Like as follows.
Army: Bonus Feat, 4 extra skill points at character creation, 1 extra skill point per level.
Each member of the Army is trained to be as versatile as possible. As such, they are the most adaptable branch.
Air Force: -2 STR, +2 INT, +2 to any two knowledge skills. +2 to Will saves.
Members of the Air Force rely more on their mind than their muscle. They look more towards the strategic solution rather than rushing in, guns blazing.
Marines: +2 STR, +2 CON, -4 INT, -2 WIS, +2 to Fortitude Saves, Free Endurance Feat.
Marines are trained to do two things; if it needs to be there, they'll movie, if not, they'll destroy it. They can pick it up, put it down, make it rubble, build it again, and still survive the fight.
Navy: +2 CON, +2 INT, -4 WIS, +2 to Will saves, +2 to Balance and Tumble checks.
The Navy is trained to be on a ship most of the year, regardless of their job. If a sailor can do it, they can do it on a swaying ship.
Coast Guard: +2 to Balance and Tumble checks, 4 extra skill points at 1st level, 1 extra skill point per level.
The Coast Guard spend their time on ships, but not as much as the Navy. As such, they're also trained to be very adaptive.
These are only suggestions, not meant to offend.