I would prefer if their type or intelligence was referenced in their decision to attack.

So... very aggressive to aggressive only when chances of winning are good to somewhat passive. I'm sure that's a ton of extra work, but hey, you asked.

Speaking as a Demonologist, some demons are very aggressive while others are quite docile. The key would be lore. Knowing your enemy is the first step towards conquering it. S in essence, there should be aggressive ones and passive ones. The trick is consistency and giving the user foreknowledge of the type. 

Maybe a message like the "Look at you wondering around the wilds like you own them," only saying something more to the tune of "A Lerath demon was spotted near your city/army. It attacked your troops ferociously, despite being outnumbered. This is certainly a demon to be carefully watched." That way we get a little info, while also having the fun of discovering a new species of demon. Later we could even have encyclopedias of the characteristics of demons in Elemental. 

While I realize this would require individual tweaking of every creature, it would be something to keep in mind if you are open to giving each creature an adjustable aggressiveness level in the XML. 

If it is more general, I vote for highly aggressive only when their home is within your influence. 

I would assume you'd want monster's difficulty level to be based on the world difficulty. Beginner/Easy level, they're pretty passive and you can just pick them off. Normal level, monsters will co-ordinate to defend their lands and launch small, obnoxious forays into your territory to destroy outposts. Hard/Flee-For-Your-Life Difficulty, the ravenous beasts will throw everything they have to take you out of the game. But maybe I'm misunderstanding the nature of the question. If you could only pick one of the above three, I'd suggest the normal level difficulty. That way, the monsters provide a presence

I also assume that we're talking about the strategic map, not the tactical one.

I read this as something different, and kind of cool. What if the level of aggression were based on the monsters in question? Say you've got a spider swarm. They're mindless, they'll just wander around and attack anything that moves. Then you've got wolves. They wander around and attack anything the pack thinks it can beat. Thieves and brigands eagerly attack trade caravans, unguarded cities, and heroes they think they can overwhelm, but are dreadfully fearful of sizable armies. Maybe you remember the Haunter - perhaps a group led by the Haunter would only attack groups with a magically potent character (or some other cool lore reason). So basic motivations would decide when and where each kind of monster category would attack.

THE SIMPLE SOLUTION!

Give players an option when creating a new game to set how aggressive they want monsters to be, overall.

Lies. 

I heartily approve of the creation of whole new worlds dedicated to torment.

MAKE A SLIDER! YES! (maybe you could change it in game too.) Different creatures acting in different ways too, ya I can see this is hard to figure out...

Player choice via a slider would be awesome or at least based on world difficulty. Bottom line: if I'm playing the hardest difficulty I want those monsters emulating a human opponent. I also like the idea that monster intelligence also serves to gauge how agressive they are (as LightofAbraxas and IronKaiser pointed out). The monster difficulty on harder settings should feel like a function of quality over quantity.

For example, those Troll Warriors you had in WOM were hero killers. When those stacks spawned I had to tread way more carefully than a bunch of dumb spiders. I enjoyed that way more than squashing arachnids.

monster difficulty should be determined by its type.

Animals should be cautious, unless they like eating people.

Maybe some monsters like certain things?

I agree, Alstein.

Best regards,
Steven.

Dragons and other very powerful intelligent monsters should attack anything whenever they think they could win. This includes cities.

Other aggressive monsters should attack whenever someone gets too close or if they wander close to a city, or if they are harassed in any way (ZOC).

Animals shouldn't attack unless the enemy is very, very weak or the enemy is too close to their lair or under special instrucions to attack for some reason.

Bandits should only attack weak targets.

Basically, I agree with the argument of it should change based on unit type.

Maybe we could sacrifice some peasants to appease the high level dragons, monsters, and demons?

Take into account that no monster is going to just randomly rampage without a reason, and that if monsters are little more than beatsticks they become faceless and pointless in the grand scheme of things.

Personally, I'd prefer a scale - between more docile (monsters individually avoid cities and push away from them, perhaps forming small tribal communities or eventual huge armies) and incredibly violent (monsters see anything that isn't them and attacks) as the two extremes.

More to the point, are monsters just there to cause problems? Or can they be used in alternate ways?

Aren't most monsters bound to an area? Do those areas spawn new monsters or is there a difference between the things protecting resources and the things that roam about?

So long as I don't get the occassional situation (as happened a couple of time in elemental) when i'd be attacked by armies of monsters that i could not by that point, have raised forces to defeat, I will not be too annoyed.

The problem is not IMHO in the AI in this instance, but the all-or-nothing nature of assaults. You either get attacked and defeated in your main city (game over in the early game) or pretty much nothing happens. If bandits would demand gold in return for not attacking, that could allow them to be much more aggressive.

But more than anything, the game really needs a siege mechanic like in the Total War games. That way, less than awesome enemy armies could attack cities and do meaningful damage (deprive income, steadily deplete the defenders over time) without attacking outright and spelling game over. If armies were required to besiege for a few turns whilst building assault equipment, it would also make it much more difficult for human players to rush the AI and move down the road to from their town to the AI's and capture it, all in the space of two turns. You'd get more interesting situations like armies from other cities coming to one city's relief. As I say, the Total War games basically do this perfectly.