I've never found AI to be a strong point in any RTS gametype. Switching of the difficulty levels usually just entails quicker movements to the next command, and a different attitude (attack, turtle, expand, etc.)
RTS, IMO, are for playing other humans and trying to decide what kind of person he is and what he will do.
I think as long as there's some story in the forms of cutscnens pre and post level and maybe minor cutscenes in the single player, it'll be decent. Since in-game there's not much room for story advancement playing a REAL TIME (everything is happening in real time to clarify) game. I don't know if that makes any sense.
Then again I won't be playing Halo Wars for its story either, I'm just hoping it has good RTS balance/gameplay.
O and I know the sound/music is going to be awesome. Really excited to see what that Wwise sound engine can do for games, looked quite promising when I saw a demo of it earlier.