I would personally gauge AI like this, if I were to develop it:
Easy = For the beginners...enemies do as CMcMahon descibes above. No fancy tactics, smaller, hodge-podge fleets, leaving own worlds unprotected, not well organized in diplomacy, trade, etc...
Normal = For the average/experienced...enemies play more consistently with average sized fleets and a better mixture of ship types than above, use some basic tactics, and are fairly smart with their economy and diplomacy.
Hard = For the experts...enemies continually have the upper-hand, i.e. use flanking (advanced) maneuvers, create diversions, etc...with large, well balanced fleets. They also can win economically or with diplomacy only, not just militarily.
Now I know there are differing game-play styles you can assign the AI (I leave them random), but a turtling normal AI should have all their planets well defensed, the same as an aggressor AI should! Just because normal aggressor AIs are war-mongers, they should have the sense to protect their infrastructure to feed their war machine, if they don't they'll have a short-lived warring campaign!
I guess I can switch a saved game to hard AI and see how much it changes the game from the way it is now. I'm by no means an expert yet and find playing expert mode in most games as playing against cheaters with game hacks/cheats enabled though! That's not much fun IMO! Even with the few quirks I mentioned above here or in the OP, the game still rocks and sucks up a lot of my time!
