it is very irriating when the enemy i just paid off a planet or techs for a peace treaty immediately declares war on me again (apparently their treaty with another faction at war with me causes them to immediately declare war again). I really like the new CIV IV enforcement of treaties. I would recommand that once a peace treaty is struck, neither side should be able to declare war on the other side for one year.
Also, the current system is very exploitable by the player. For instance, i offer a losing AI a peace treaty for a ton of techs, then simply declare war right away and finish him off (scoring a bunch of free techs).
Fix this and make 'borders' stop offensive fleets and this game would rocket to the top of my list