One thing I'd like to see improved is the unit AI, which is frustratingly poor. This would probably annoy some serious multiplayers, but that's not necessarily a bad thing.
The HD Edition has a new AI[0] that's much stronger than the old one. It's built by Promiskuitiv and Archon, two well-known AI scripters in the Age of Empires scene :)
As I understand it, this is a bot AI, and not a unit AI. Am I wrong?
How does the new AI differ from the original one? I don't see it in the change log.
There are custom (modded) AIs available that you can download. I've heard that the Barbarian AI is meant to be able to give even seasoned players a serious challenge. I've also heard that a much stronger AI has been bundled with the most recent expansion.