First off, even as weak as they are, I get lots of value from naval units (mostly because they go unchallenged, and if I can get them to +range picking off random units/avoiding cities, then I can join in the city siege as well). That said, they are on the farthest edge of the pyramid, making the techs a little less versatile (since fewer techs require them as prereqs), so a little upgrade might be acceptable. Bonus upgrade really should also give a bonus to buy units also. As for other abilities being too powerful, I like powerful for special traits, so instead of thinking of reducing some, I think we should look at how to improve the others to match their power. The biggest comment I agree with is AI diplomacy when it comes to war. I devastated my opponent's army, took 3 of his cities, and he still demanded I give him half of everything I own in order to 'buy' peace. After taking out almost all of their cities, I finally allowed them to have peace for nothing (which disappointed me, since they ought to have given me everything they own), mostly because I wanted them to build more cities for me to puppet (I was trying out a cultural game, and puppets are a great way to gain free culture without impairing the policy upgrade costs). Even after that, they still call my demands ridiculous, and tell me I don't have the army to back up my threats.