...I really do hate "allegiance", "happiness", or any other metric of populace in 4X games that is so strong that it dominates play. I'm looking at you, Civ series.
Do you feel the same about Civ4 as the previous versions?
If it is done so that I don't have to spend most (or even a large plurality) of my time keeping my empire from splintering with bread-and-circus type pandering, I'm okay with it.
My hope and plan is that much of the determination of species' allegiances to empires will be based on the alignment scales discussed in this thread, which are not micromanagable. Happiness would be determined by a larger variety of factors, likely including some "pandering", but I don't expect the means of doing this micromangable either, so we shouldn't have situations like earlier civ games where you have to constantly check all cities to make sure they haven't fallen into civil disorder and pick a worked tile to turn into an entertainer.
Also, If the idea here is to make the war and peace decisions of the player depend on some sort of rationale or other in-game influence, I have another approach.
If you and some empire share a lot of values in common (or nearly so), you can trade, be friends, etc., but you can't declare war on each other without a damn good reason (cuasus belligeratus). The inverse is true for nations that have opposing cultures.
Restricting player choices ("you can't declare war", "you can't declare peace") depending on the game situation isn't really the same thing as giving reasons for players (and especially AIs) to prefer a particular action. I'd rather try to achieve diplomatic consistency through rational motivation than game-imposed restrictions, if possible.
Further, you should be able to change your cultural alignment over time using various methods (actions, buildings, techs, whatever), or similarly change other empires' alignments. Maybe there will be a natural drift towards agreement of culture between neighbors.
Other than the natural drift, this is consistent with the current alignment system.
The AI-informing implications of this are obvious. To the player, it should make sense if some empire declares war, because they probably hate you already for cultural reasons.
This is one of the goals I have for the alignments / allegiance and happiness system, except that empires don't hate empires; rather species hate empires, and empires will be motivated to chose particular diplomatic actions with or against other empires based on how the species on planets in the two empires will react. For example, if most of your planets contain a species that loves another empire, you declaring war on that empire will cause large happiness penalties, so you are strongly discouraged from doing so until that species opinion of the other empire changes.
A major reason I like this system is that it allows all the mechanism to take place at the empire level. I don't have to worry about specific planets' happiness, allegiance, or what have you.
The outcomes of the stuff I described above will be largely determined by the allegiance of species to empires. Each species has only one allegiance for each empire, so as long as a player know the number of planets that he or she controls that contain that species, and the species' view of another empire, the player will have a good idea what the reaction would be if a particular diplomatic action were taken.