I always found it's too easy to be diplomatic in GC2 (only played with expansions though). I have what I consider a bad habit of playing with diplomacy and researching tech instead of going to war, and in GC2 it's too effective, as it's in Elemental. It's a bad habit because the diplomacy can still be played against the AI more easily than straight war, and usually no AI race will be in good relations with others. It's too easy to become a superpower in peace.
I haven't yet seen any aggressive AIs in Elemental, they're mostly just sitting there. In my opinion an aggressiveness slider would be an easy solution (if the AI really is that aggressive), but in an ideal world there would be techs, spells and diplomatic options that allow a more peaceful playstyle. It would be cool if there were spells that would defend the caster's area of influence by weakening/attacking attackers, or maybe supplying magical defenders to any town that gets attacked.
As mentioned in earlier posts, the diplomacy needs more options. I don't like straight cease-fire/peace/alliance, they're too powerful if AI goes for them easily. There should be options for tributes, conditional peaces and such. Peaces without drawbacks tend to favour the player, players can more effectively force a peace with everyone, or any opponent in position to threaten, and then build a huge economic and technological advantage and steamroll opposition, or win with a Master Spell without having seen a war. I often tend to win this way in GC2, and even in Sword of the Stars (which is WAY more about war) I sometimes end up being left alone, and then winning easily. I don't like that sort of wins, they require no effort.
I'm all for more peaceful playstyles (in addition to war) as long as they aren't clearly the easier option towards victory. They should require different focus and playstyle, but still be complex (and rewarding) to achieve.
Personally, I tend to enjoy most the victories that end up burning most of the world/galaxy with few left alive. I'm really disappointed that the star killer thing in GC2 is so ineffective, I want to destroy worlds 