Isn't rainforest an utterly horrible map for domination? Lots of land, slow start and low production. QUOTE]
So... maybe you should just use slavery? Actually I haven't played any domination games on rainforest, just SS and cultural games, but I could imagine that there are some advantages of this map for domination victory:
1. Most of the iron and copper can be found in desert regions of the map, so AIs without deserts might be without either copper or iron.
2. AI have to build a lot of workers, often 4-5 turns up by a new city what you can easily conquer and then make peace if you are not strong enough.
3. Most of all, rainforest is a fun map to play!
Every time this subject comes up, it's all about military victories, space or high score. No love for UN.

Seriously though, there is a lot of air in the UN dates of the deity tables, larger map sizes. And it's also a victory condition that is more accessible on faster game speeds for the less experienced player. Lately most deity gauntlets have been marathon, which call for very different tactics than normal speed. Would be interesting to follow a normal speed large or huge game for once.
I hate UN. It is all about how AI deals with you, and sometimes they mess up with you even if you do your best to get the highest diplomatic values.
Cultural games are fun, though. I prefer cultural games over military ones.