Neutral units are controlled by another player, not by the AI.

Could you explain what "ghost mode" means? Are you talking about Heroes 5?

Here are some minor details:
Revealing the map (although you won't see it as a hero, you will see it as a ghost, and you can walk through enemies)
Cursing an enemy hero (if you upgrade the skill, you can reduce the hero's stats; however, it can only be dispelled by another ghost if that ghost has the corresponding skill)
Cursing mines (poof, and the mine stops working)
Controlling neutral creatures (if the skill is upgraded, you have a better chance of having neutral creatures fight for you against the enemy)
Dispelling negative effects from other ghosts (if not upgraded, it's difficult to remove the curse)
Attacking other ghosts (when two ghosts collide, the one with the stronger attack wins; the loser loses one of their skills and respawns at the capital)
Also, ghosts have nice icons (it's a shame they aren't in color).