Well, the AI for the Campaign and the AI for skirmish had very different goals...  The campaign was all about telling the story where you are *really* fighting against the covenant. 

For skirmish games a big focus was made to model the way the AI thinks after a human player:  thinks about one thing at a time, and can only act on one thing at a time.  We realized early on that it is really easy to make an RTS *hard* if you just let the computer micro every unit every second to do the right thing.  Playing against this AI was challenging, but it was not fun to lose to being out micro'd in such an impossible way.  To make the AI play like a human we built in the systems where the AI has to spend time thinking about different things, and have different quality values for what it did think about.  The automatic AI difficulty is the backbone of this system and it drives tens of equations that set things such as time required to act on something or how smart the AI is at choosing counter units etc. 

To make a long story short, the two systems are so different that the automatic difficulty used in skirmish did not get applied to the campaign.   For replay value in skirmish we have auto AI to make sure the challenge keeps going up, and the ai communication system so you coordinate tactics with the ai as if it were a human ally.  For campaign replay value, co-op, skulls and trying to get a high score on the leader board are areas where we spent effort to add replay value. 

first@ Xexyl:
You and your team delivered a absolutely great game-I love Halo Wars.

To the question:
I play only on legendary, because if you are familiar with PC RTS the normal and heroic difficulty are too easy. If you are a newbee it's ok. On legendary you will definately have a challenge,even if you are a pro RTS gamer you have a challenge to beat every level with a gold medall-trust me, it's not too easy.
... it is too short, but then you can go with the great multiplayer.

...at least in multiplayer-matchs you will find your master.

Xexyl - Great job on the game. I am really enjoying it.

its hard on normal but then again i just suck at it

That is typical for Halo game.

The campaigns are very accessible and you need to find skulls and black boxes (both appear on every map after some decisive secondary actions).

Once you activate the skulls, it dramatically change the behaviour of the game (for HALO 3 I mean, I haven't collected enough HW skulls yet) and boost the difficulty of the game.

EX: Some skulls give up to +50 extra power and +25 hit point (defense) to the skirmish rebels. It has a big impact on the tempo of the game because you need to have a significan t upgraded task force to get rid of them, and claim a ressources/extra base spot.

So the interest in the replayability of the game is to focus on those objectives.

I don't know if it gives any rewards for that (like the armors of HALO3)... I bought the online prima guide but haven't seen anything about it.

Xexyl, I just wanted to thank you for the link to your blog earlier, there were some fantastic tips and guides in there for Halo Wars, the AI control/clicking/flares is superb but not sure why this isn't in the manual as not many people must know about it?

It adds soooo much to the skirmish games!