That one's a really good idea actually, a society focused entirely on farming doesn't produce lots of trade goods.
*nod* -- agriculture economy means that your economy is built around food, food and more food. Not built around people doing things with the food -- just more food!
+50% trade route yield
-10% gold
+20% culture in all cities
Other civs get -happy for not having foriegn trade
+50% war weariness
Don't like this one. -happy for foreign trade was in for a while, it's difficult to justify though and not fun especially as another civic already has this mechanic. I'd rather this civic have only positives, as trade route income is not 100% reliable if your neighbours close their borders.
-happy for others is easy: your society is getting lots of goods from free trade, theirs isn't.
The only negatives are war weariness and the -10% gold. Other than that this is a kick-ass civic.
The -gold is a minor detail: the trade route income will more than make up for it, if you have good trade routes. If you don't have good trade routes, then this civic becomes a bad choice.
And if you are an expansionistic warmonger, this is also a bad choice, even if it has large economic benefits for large empires.
So we get civics based on what phase your economy is in:
Agriculture:
+2 food -2 health per farm
+2 health granary and storehouse
-50% trade route yield (!)
+50% iNumCitiesMaintenanceModifier
This is great for an early economy -- gets you off the ground quickly. As your empire grows, it gets worse: agriculture is a poor way of holding together the economy of a huge nation. You need something more refined.
Conquest:
+2 XP
Food for Units
Gold from taking cities
This slows down your growth a bit, but is otherwise a good warmonger civic. You can turn breadbaskets into military factories.
Merchant Houses:
No foriegn Trade
+50% gold
-25% lightbulbs
-2 happy theives guild
-50% military unit production
Can rush production with gold
+2 gold per unit maintenence
A builder civic. Making war as merchant houses is hard -- but possible, given the massive gold bonus. Gold rushing
and +50% gold is crazy synergy. The gold cost of units, the penalty to military unit production, and the lightbulb penalty prevent you from being an unstoppable force.
You could probably make a military empire using this -- but it would be one of elite units, not a swarm.
Foreign Trade:
+50% trade route yield
-10% gold
+20% culture in all cities
Other civs get -happy for not having foriegn trade
+50% war weariness
Best for peaceniks and tech mongers. Being at war while under foreign trade hurts badly, but it lets you stay a huge empire -- it just doesn't help you get the empire!
Each one is clearly better in it's area of expertise:
Agriculture: Small city population booms.
Conquest: War
Merchant Houses: Building and developing cities.
Foreign Trade: Technological and Cultural growth.
I could see a civilization moving between them from era to era. Agriculture early on, to get the initial population boom. Conquest to increase the empire size. Merchant Houses to build up the newly conquored territory. Then Foreign Trade to slingshot into the later part of the tech tree, or to new military technologies, to start off a new wave of Conquest...
Note that Agriculture has the neat property that +health aliases into +food very quickly. It never gives you less growth than other civics -- 2 sickness is not as important as 2 food. If you have a health surplus (from whatever source -- varied diet, buildings, etc), you can grow fast and big.
And your population won't crash when you grow out of it.