What effect do difficulty levels have?

There are three different difficulty levels (Easy, Medium, Hard). What kinds of penalties/bonuses do they cause? For bonus points, how does the "Aggressive AI" option play into things?

I don't know all the details yet but this is what I've seen so far.

Easy

• Less Neutral spawns for some campaign missions (Free Resources, no guardians)
• Lesser Amount of Neutral spawns than Medium difficulty
• Timid AI, gathers little resources

Medium

• Lesser Amount of Neutral spawns than Hard difficulty
• Slightly Aggressive AI, gathers some resources [but I've noticed that they tend to stay idle or make beelines for your base or hero. I've only observed this but can't draw any concrete conclusions.]

Hard

• Aggressive AI, actively gathers resources across the map (which, amusingly, causes him to lose some of their units making them weaker in the long run)

Based on what you can see by going to the custom difficult settings, difficulty level affects four things

• Starting resources
• Number of neutral armies
• Growth rate of neutral armies
• Aggressiveness of AI

Each of these has 3 levels to it. Easy sets them all at the easiest level, normal at the middle, and hard sets them all at the hardest level. Custom difficulty allows you to combine various levels for each setting (so you could have aggressive AI, but reduce the growth rate of neutral armies).