Just beating missions isn't difficult, but if you want S ranks, that takes thinking. And the unlockable hard campaign will severely punish any mistake you make.
Yeah, beating the missions is challenging, but the replay value comes from perfecting your strategy and getting those S-ranks. And then getting the S-rank for your whole campaign. I think the highest I got was an A-ranking for a campaign.
Yeah anyone can get through Advance Wars without much trouble on normal mode. Hard is a bit more challenging though. Then getting S-ranks is even more challenging. But the real challenge comes in the bonus maps. Some of them I think may be literally impossible. I mean, they're PROBABLY beatable, but I sure don't see how.
And of course if you're just so amazing that none of that is a challenge, you can always build your own maps that will kick your butt.