Any building that does nothing aside from allowing you to build another unit. They should be removed or flushed out to allow them to do something other than slow down your production and confuse the AI.
Useless summons, such as the Azer. If a spell has no flavor or usefulness it should be removed or reworked.
Unending vassalage Requests. You should never be approached to vassalize a state with a different alighnment unless it has the same religion as you do, and you should not be approached to do so constantly.
Privateers. They are overused by the AI, consider making them a national unit.
Elephants. They do nothing but slow down the early game and take up slots that should be used to spawn animals that can be defeated/subdued.
Great Commanders. Give us Great Generals instead.
Lengthy, all consuming fires. Give fires a much higher chance of burning out rather than spreading farther. I have used a single adept to put entire elven empires into chaos.