In my inpression, GalCiv3 favors the attacker enormously.If someone attacks with a real fleet (20 to 30 points of logistics), he will almost always meet no real resistance from the AI.
It is also quite difficult to fortify a planet against a real attack. Even if I have "planetary defense" maxed out, my planet still gets conquered easily by a single transport.
Master of Orion solved this better by far. You could build up planetary defenses, as e.g. planetary batteries, fighter squadrons, star-bases that fought against fleets and so on. These planetary improvements were able to repel at least small fleets of attackers on their own.
I think, having some real, working planetary defense would improve the game and would make life easier for the AI.
What do you think?