Well, let me dip my pen in this ink well and see what I can write up. I'll try and keep things to gameplay, and talk less about historical accuraccy.
The Glory of Rome (Rome): +25% production towards any buildings that already exist in the Capital.
This ability is just flat out good in a very raw kind of way. Anyone can see the usefulness of a 25% increase to producing buildings. This fits in with how the Civilization franchise has always treated the Romans. They usually end up being the most basically good.
The Great Warpath (Iroquois): Units spend only 1 Movement Point entering any tile with a Forest.
I love this ability. In Civilization 4, I would play Aztecs and rush straight to Jaguars. They could be build without Iron, had a power of 5, and started with woodsmen 1. Kill a few barbarians and suddenly you had a unit that could double move through forests, and attack out of them at 50%, or more, I can't quite recall. This lead to some amazing raiding/marauding tactics, and if you had even three Jaguars in an enemies borders, you could effectively shut a civilization down permenantly. Being able to do this with an entire civilization right from the get go sounds super super super hot to me.
Of course, the long term benefits begin to wane. Even as you go through the midevil era, forests become less and less prevailent. But, I argue, that the early game benefits are so vast, that it hardly matters. You can straight up obliterate civilizations with this ability in the early game. I promise.
Manifest Destiny (America): All land military units have +1 sight range, 25% discount when purchasing tiles.
Another, straight up, just good ability. The extra sight is going to allow troops to respond quicker to threats, and the 25% drop in price to purchase tiles means faster cultural expansion, and more gold in the pocket to spend on other things.
I suspect that the bonus to purchasing tiles will become less useful in the late game, but the early game benefits are obvious, and the 1 to sight will always be nice.
Trade Caravans (Arabia): +1 gold from each Trade Route, and Oil resources provide double quantity.
I don't think we know enough about trade routes to know if this ability is awesome, or lame. Double of any resource is always rad though. At any rate, it seems to me like this is an ability that is largely useless until the mid to late game, when it really starts to kick in. If I were playing Arabia, I'd be deathly afraid of hit and run tactics, and having my trade routes pillaged.
Ultimately, we'll simply have to see exactly what's up with trade routes before we know if this ability is any good.
Sacrificial Captives (Aztecs): gains Culture for the empire from each enemy unit killed.
I like the elegance of this ability. It obviously forces the player into a more militaristic mindset, but you DID pick the Aztecs. I like that you can simultaniously be waging war, while also being quite cultural. Winning a cultural victory as the Aztecs while killing all around you would be quite amusing.
Art of War (China): Effectiveness and spawn rate of Great Generals increased
This is entirely dependent on how much effectiveness and spawn rate we're talking about here. There's not alot of info on what Great Generals will do in Civ 5. Conceptually it seems very good though. Who doesn't want more things that kill things.
Monument Builders (Egypt): +20% production towards Wonder construction.
Just a good ability. Of course, maybe wonders are lame now. ;P
Sun Never Sets (England): +2 movement for all naval units.
This is one of the two abilities I'm most skeptical of. It does speak to a level of confidence that the developers have in their new naval combat design, but I am trepidatious. Naval combat has always been poor in Civilization, and while it seems like that might not be the case any longer, a simple +2 to naval movement might not be good enough to rival things like 20% wonder production, or 25% less to land purchasing.
Ultimately, the Zone of Control might moot the benefit of this ability. But, one can never discount the usefulness of simply getting units into combat quicker.
This ability does raise a question for me though; Does moving OUT of a zone of control still constrict you? If it does not, then this ability could be quite powerful in regards to manuevering your ships around on the battlefield, or even having transports escape faster.
This has a huge "Wait and See" on it for me.
Ancien Regime (France): +1 culture per turn from Cities before discovering Steam Power.
This ability tips the hand a bit in regards to the developers being totally find with these civilization abilities becoming less useful during certain ages. Heck, this one simply stops working outright.
Again, we won't know exactly how good this gets until we know how much things cost culturally. What we can say is that this ability will get better the more cities you have, and so an expansionist strategy is more beneficial while you're playing France. Simply put 6 cities is better then 3 as far as the ability is concerned.
Furor Teutonicus (Germany): Upon defeating a Barbarian unit inside an encampment, there is a 50% chance you earn 25 gold and they join your side.
This is the second one I'm skeptical of. This depends ENTIRELY on how prevalent barbarians are in the game. If barbarians exist how they have in the past, this ability is flat out bad. Awful to the point that I would make fun of anyone who chose germany to play.
If Barbarians are quite present though, i can see how this ability would be very interesting. But no matter how you cut it, this ability is very very specific, and loses any punch it might have in the mid to late game.
The hope is that, in the early game, germany will get enough gold and extra units from this ability that it will be able to carry on into the mid to late game with some momentum.
I am extremely skeptical.
Hellenic League (Greece): City-State influence degrades at half rate and recovers and twice normal rate.
Another just really solid ability. Getting the social policy that effects City states along with this will make this an incredibly powerful ability, especially for anyone going for a diplomatic victory.
Population Growth (India): Unhappiness from number of cities doubled, Unhappiness from number of Citizens halved.
I don't want to say that I'm skeptical of this ability, but rather that I don't see how this is an ability that is an actual benefit. I believe that this is mainly because I tend towards military strategy, and expansion, and so I'm having a difficult time conceptualizing this ability.
What the ability will do is encourage players to have less cities, that have more population. Is this a good thing? I assume there are players that will say yes.
I suppose alot of it will come down to exactly how much unhappiness is produced from the number of cities and from the number of citizens. If, in the end, I can actually get more citizens from this ability then I could otherwise, then I see the merit. If it just ends up being a redistribution of numbers that end up being the same as before, then I truly do not see the benefit of this ability.
I am, however, extremly open to someone creating a case for it. Perhaps there's something there that this old warmonger hasn't quite captured. To be totally clear on why I'm not skeptical of this ability; I feel that the ability may very well be a good one, and that I am simply unable to see it's usefulness. This is a limitation of my own, and not a limitation of the game, I am sure. The ability is just not for me.
General Notes
It's incredibly plain that there are abilities that start out strong, and then lose their effectiveness as the game goes on, or that start out weak, and gain in power as the game goes forward. I don't think this is a bad thing at all. What it will do is force players to utilize the abilities they have to their full extent, while they can.
I don't see many people playing the Iroquois and playing a passive early game, for instance. But you could also say that you'd never see an Egyptian not try to build Wonders. Or you'd never see an Indian player expand quickly and broadly.
Just because an ability doesn't stay useful for the whole game, doesn't mean it won't be overwhelmingly good for the period that you can use it. That Iroquois ability is going to straight up ruin civilizations all day long.
I'm so playing Iroquois.