I'm no where near as experienced as Sisiutil, but I've been playing MP pretty consistently for a few weeks now.
I rated the traits (and civs and leaders, in the same post) here:
http://forums.civfanatics.com/showthread.php?p=5826451#post5826451
Specifically, here are my trait ratings, from an MP FFA Pangaea perspective, with an emphasis on warmongering quickly and powerfully:
Agg +3
Chm +2
Cre +1
Exp +1.5
Fin +2
Imp +2
Ind 0
Org +0.5
Phi 0
Pro 0
Spi +0.5
Please feel free to disagree/comment/critique/provide your own ratings for MP either here or in that thread
I have a penchant for enjoying analysis
Here's a bit more on why I chose to rate the traits this way.
Agg +3
Aggressive is a free promotion, one that doesn't take XP. It also reduces the cost of a Barracks, which you'll want in practically every city (there are times when you must go 100% military production -- e.g. you are being attacked and need to push the enemy back).
Chm +2
As you're going to be doing a lot of fighting, cheaper promotions means you'll have stronger units -- as well as being able to better counter enemy units by having more promotions available faster. As most MP fighting takes place outside of cities, City Raider is of less use than in SP, making the Cover/Pinch/Shock promotions and their like all the more important -- Charismatic lets you get them that much faster.
Cre +1
Creative isn't bad, but it's not as strong as the other traits -- while pushing an opponent's borders out is very doable early on, and forgoing a monument to get a BFC is helpful, Creative's strength starts to plummet right around Construction. Those wide cultural borders are no help at all vs. an enemy stack complete with catapults to tear down your carefully built up cultural defense. Cre is great for making an early land grab, but unlike the AI, enemy human players will not tolerate your borders slowly pushing into them -- they'll go take the city instead and Creative itself does little to help counter anything they can do.
Exp +1.5
Expansive is strong because of the 25% worker creation speed, and that worker advantage matters a lot early game -- the bonus health also lets you skip building aqueducts until a bit later, meaning you'll be better able to focus on your military. And since Hereditary Rule already takes care of Happiness issues, you'll be able to grow happier and healthier cities than everyone else, working more tiles. Do not underestimate the 25% worker production, since early game workers matter so much.
Fin +2
Most MP games are CE based, since SE is just too micro intensive for the Blazing turn timer that majority of MP games are played with. Cottage spamming (with proper military forces to catch and kill any potential pillagers) along with coastal tiles makes Financial get a solid tech advantage thanks to its increased commerce. And since there's typically no tech trading in MP, that solid tech advantage means you can build better units faster than your opponents.
Imp +2
Faster settler production is handy for getting your second, third and maybe fourth cities out that much faster. After that though, you'll likely just be taking enemy cities. A good thing to keep in mind, and one that I learned the painful way, is only settle great cities in MP -- don't make a lot of little mediocre cities. There's nothing wrong with leaving bad land unsettled -- in fact, it can invite an over-expansive opponent to settle there, and the maintenance costs based on # of Cities spikes up pretty quickly on Small, so you need to make every city count.
Finally, Imperialistic provides 100% GG emergence, which means you can get Military Instructors into your top production cities all that much faster, giving you better promoted units. And 100% GG emergence is a benefit that starts early and lasts all game, thus solidifying the power of the Imperialistic trait.
Ind 0
While faster forges are nice, you won't be building wonders in MP FFAs... Why not? Because building wonder incurs a huge opportunity cost in terms of military, and if someone else builds a wonder, then you can go take it from with all the troops you've built while they were spending time building a wonder.
Org +0.5
Games are typically decided before Organized really gets to shine -- Organized starts its full power when you have a large empire with expensive civics, but most MP FFAs are decided by Grenadiers at the latest, so empires and civics just aren't that expensive yet. Courthouses, however, are nice, especially how they help with EP now.
Phi 0
Unless you're a micro god and and run an SE on a Blazing turn timer, I'd avoid Phi.
Pro 0
Protective, while it can have some uses with gimmick archer rushes, is primarily a defensive oriented trait. The problem with archers in MP play is that while they're great for defending cities, they are horrible for defending land -- what do I mean? You can have a stack of archers in a city, but they're not going to be able to go out and stop enemy stacks from pillaging your land to nothingness. Protective enhances archers, and archers just aren't that good in MP from what I've found.
Spi +0.5
Spiritual is handy for early civic switching (Slavery, Hereditary Rule, Bureaucracy) but there really aren't that much civic switches you'll be doing, since you'll likely be at war or at least on war footing all game. Also, by time time Spiritual really shines -- when you have a nice of civics to switch between -- most MP games are long over, and if they're not, you can (ab)use Golden Ages or even build Cristo if you have the hammers to spare and have a secure military situation.
