Civ V could use a war penalty, a la war weariness
The problem without some sort of cumulative (or constant negative) penalty for being at war is that an aggressor who is starting to steam roll an opponent has ZERO incentive for peace, which makes the weaker player even that much more weaker. There were plenty of games I have played in Civ 4 where I could have kept attacking my opponent (or AI), but asking for peace was a better immediate alternative for me due to the amount of unhappiness I had accumulated from fighting so much in his territory.
Also, SDI. Civ V needs a possible defense against abombs and nukes.