Saw this on another board and this guy hits the nail on the head.

1. AI does never specialise its planets, i.e. no planets that only have factories to produce ships quickly.

2. AI does spend inane amounts of money and resources on static defenses, i.e. the ships in orbit. Mobile warfare is much cheaper though and with unrestricted movement it's definitely advantageous to have ships moving around in fleets and meeting the enemy in open combat than waiting for him to assemble a superior fleet an blasting you from orbit. I can easily achieve local superiority even with numerical inferior units because the AI doesn't move most of its ships from orbits.
3. AI doesn't seem to know what starbases are good for. It builds the occasional economics starbase, but it doesn't seem to cluster several of them together with its best planets. On top of that it seems to pick the location of starbases at random and doesn't seem to try to get the bonuses to as many planets as possible.
4. AI doesn't always go for those resources as fast as possible. It sometimes builds an economic starbase first? Which is obviously a bad choice as an economics starbase may influence like 5 planets at maximum while the resources give an empire-wide bonus. Bad priorities.
5. AIs never make alliances against me. Really, when I am the evil empire that has conquered half the galaxy, everybody else should gang up on me and not attack me one at a time. That's a bad movie-villain tactic that always backfires They even made alliances like that back in Civ2 (or 3?) and it certainly serves to make the end game interesting as you have to win the last big struggle.