Had they gone in the advertised direction of "Chief vs. Locke", I feel like the message would've been "what do you do when your hero goes rogue?" Questions about how much power the Chief actually deserves and who, if any, should trusted with it.

The current version feels two themes. The first one is: "wow, the galaxy sure has changed a lot since the end of the war". That half I like more, and is the most detailed. The second theme is: "what do you do when your girlfriend didn't die and now wants to conquer the galaxy? I dunno, we'll find out next game." That one... bleh.

I agree with what you said about Reach having a clear theme. Like it, dislike it (I like it), but I feel like it had the most consistent story theme throughout.

I guess the other games could be summed up as:

Halo CE: "You're trapped on an unknown world surrounded by enemies. But you're a badass supersoldier. Fight your way out."

Halo 2:
Master Chief: "We're gonna blow up the Covenant and save Earth and... oh wait, I'm on a ring again."
Arbiter: "Perhaps what you've been taught all your life isn't the right path after all."

Halo 3: "End of the world? Heck no! We're saving this galaxy!"

ODST: "Things look dark, but you'll make it out as a team."

Reach: "Even when it all looks grim, even when victory's proven impossible, even if all your friends will die, even if it costs your own life too, NEVER give up."

Halo 4: "Sometimes you can't save everybody, even if you always have before."