Well, I have IG and like Zerg says the diplomacy and technology tree are ideas that seem taken from IG and made even worse!

The campaign AI in Imperial Glory is better (much more cautious, build up sometimes much better than you, have the brains to even out their armies, in diplomacy : they offer you some good deals and certainly don't reject the more strange ones, Peaceful annexing is a good feature to build up your empire without declaring a new world war etc...). For that reason the campaign seems more challenging and you can't just blitz your way to victory so easely it hurts.

The battles in IG and ETW are incomparable because of the age difference. It's only natural that ETW has better graphics, more options etc... when it was made 4(?) years after IG. Imperial Glory Naval battles are the spitting image of the ETW ones (apart from graphics) so again CA are hardly making an effort.