What sort of at-wills? I'd like to see Inspiration as an at-will, for instance.
I was thinking more of stances for the Strong and Tough and the movement powers that they used for Rogues for the Fast. For the other three, I wasn't sure. I could see them having more traditional at-wills, but with more of a non-combat focus for some of them. Maybe Charismatic being more akin to Leaders or causing status effects instead of straight damage. Thoughts?
Seems like careers are more like advanced classes than occupations then.
I was seeing them more as a supplemental class, maybe actually defining the character's role. I think Dark Sun added something similar, but I honestly haven't looked into it yet.
Okay so far. Especially the Academic idea. I think skills-based classes that actually do something when the GM is unfamiliar with the real-life skill in question would be awesome.
I agree. I would love to see some classes that are not combat oriented, but can do other things in combat instead of just dealing damage and would be very useful outside of combat. I see the charismatic character being a unique form of controller that was more about status effects and maybe some psychic damage from powers like Vicious Mockery.
Now I don't know how much I like this idea. Imaging picking an ability score for the Martial Artist. Do you go Strength or Dex? No matter what, you tick off half the people who want to play the class. At least d20 Modern had Weapon Finesse. Same with the Soldier... the way it was written, while it was generally used with ranged weapons (the setting, plus how ranged weapons didn't get benefits the way melee weapons did) it
could be used for melee weapons or even martial arts. And if someone wanted to be special forces, that split between Strength (rifle-butting) and Dex (shooting) and either (knife-artist... which do you use?) would still come up.
Someone is going to be ticked off no matter what you do. Scroll through nearly every forum topic and you will find at least one.

I think the best solution would be to just use feats like Melee Training that let you use another stat for your attacks and not worry about the logic of it. It doesn't really make sense that the cleric uses Wisdom to attack with a divinely powered mace, but that's how the system works. I've seen some systems like e20 that actually have ranged attacks using Strength or Constitution instead of Dexterity. It you can get around the weirdness of that idea, it could be made to work.
Multi-classing works differently between Modern and DnD 4e, so this would depend on how exactly you handle multi-classing. For instance, to use the Martial Artist and Soldier examples above, if you wanted to play a Martial Artist (or Swashbuckler for that matter), a combination of Strong and Fast gave you an optimal and flavorful combination. (Especially since you could use Increased Speed with Flying Kick!) Toss in a bit of Charismatic if you like for a swashbuckler. For the soldier, what if someone wants to be a military officer? There's going to be a lot of stat splitting going on, and abilities based on Strength, Dex, Int
and Cha!
I actually liked how d20 Modern encouraged you to do a lot of multiclassing, but 4E kind of stripped that away. Yes there are feats for it, but that is fix that just never worked for me. I think the hybrid classes were a better step, but it still doesn't have the flow of the old system. I can think of a few ways to handle multiclassing using the careers, but no matter what it will be a deviation from standard 4E.
I guess I'm saying, your ideas look a bit more restrictive than the way Modern handled things. Modern handled some things like martial artists and military officers better than 3.x precisely because it was so flexible. (Of course, the flexibility allowed you to make a completely gimped character.)