Hmm... quite a tough one, especially given how quickly the mod is evolving at this stage. Nonetheless, I'll give it a shot;
BEST
5. New Religions and Corporations: I've always wanted to see more representation of "modern" industries such as defense contractors, telecommunications and tourism. Do wish there was a vehicle manufacturing company, though. I also like the guilds, and the additional dynamics they add to the more primitive economies of the Medieval era. New religions are a big plus as well; it always struck me as odd that Confucianism and Judaism got more representation than Hellenic Paganism or Naghualism.
4. Revolutions: I may be the odd one out here, but I actually like dealing with political instability. It helps prevent the steamroll that usually ensues when one player gets a domineering score and good resource base. Look at how many "top score" civilizations throughout history have either gone extinct or are mere shadows of their former glory: Mesopotamia, Egypt, the Olmecs, Persia, Greece, Rome, the Maya, the Byzantine's/Holy Roman Empire, the Vikings, the Inca, the Islamic Caliphate, the Ottoman Empire, the Normans/Anglo-Saxons, the French, the Italians and Papal States, the Portuguese, the Germans, the British, the Japanese, the Soviet Union and now the United States. Glory is fleeting, and so is cultural hegemony; my only complaint here is that plagues haven't yet been fully implemented to properly restrict population levels, particularily from the Classical to the Industrial era. Also love dynamic civ names; really gives a better feel and helps to differentiate civs and their civics quickly.
3. Cultural Citysets/Units: There is some precedent for this in the field of real time strategy, but how often is it you see two mechanically similar settlements from two different civs in a turn based strategy game look so radically different? It greatly aids immersion to not have, say, the armies of Ethiopia populated by Europeans, or for Greek spearmen to far more resemble phalanxes than medieval pikemen, or for Tokyo to not in any way resemble Barcelona.
2. So Many Civics: A carry over from RoM, I'll admit, but you guys have really boosted up and polished this aspect of the game. Outside of a Paradox Interactive game, where can you see such a detailed and interactive representation of the societal and political divisions and competing philosophies within many of history's greatest civilizations? Where you can see and experiment with the subtle yet radical differences between, say, aristocracy and plutocracy? Where you can found proto-Marxist utopias in Ancient Rome? Where there is more to economics than barter, mixed economies and communism? Where immigration is actually represented as an aspect of national policy?
1. The Sheer Volume and Quality of Added Content: Do you know what I despise more than almost anything else in the world? "Sequels" to simulator or strategy games that actually reduce the volume and quality of their content, replace genuinely funny and warm humor with debased, adolescent vulgarity, and turn beautiful isometric 2D environments into bloom-ridden, uncanny valley-esque android vertigo nightmares that have all the entertainment value of a quantum physics lecture and all the challenge and complexity of a boy band song. My hat is off to any and all modders who work to keep real, quality games alive, and to turn gems into diamonds.
WORST
5. No Blue Marble: A bit of a petty complaint, I'll admit, and I understand it would strain those with lesser computers and maybe muck up the new terrain, but part of me just loves the sight of a nigh-photorealistic monsoon region.
4. Prehistoric Era Doesn't Quite Feel Right: This would be higher up the list were it not in the process of being fixed. I'd like a Prehistoric era where people move around a great deal. Where, for instance, Indians migrating to Australia or Siberians to the Americas can actually happen without a great deal of scripting. Where nomadic civilizations such as the Mongols, Timurids and Huns can play in a radically different manner to sedentary civs. Where competition for scarce resources during ice ages is fierce and prehistoric megafauna and neanderthals are a constant threat that encourages creative thinking, such as building on the edges of deserts to avoid the nastier beasts and putting the hurt on any enemies who try and scout or wander by your little camp.
3. Loss of UN and Guerilla Units: I very much liked the UN mod for RoM, the one that enabled the production of UN units at UN Missions that only the Secretary-General could use, and only defensively. Encouraged some interesting strategies and provided a good representation of modern day peacekeeping forces. Also miss the religious guerilla (I'd love to see organized crime and terrorism represented in a similar manner to WLBO, or see them as negative "corporations") and the Technical. There's something incredible about having a legion of toyota utes at your beck and call, but I can't quite explain it.
2. Inadequate Representation of City-State Periods and Divisions: Take a look at, for instance, the various squabbling cities within ancient Greece, the Warring States Period in China, Renaissance Italy or the Sengoku period in feudal Japan. Germany was only a united nation and culture relatively recently; a Prussia, a Hanover, a Bavaria would do it some good. Basically, civs which are the first to split off from Revolutions and share the same basic culture as the civ they split from, with civs with similar cultures uniting in times of war, much as civs with similar religions do.
1. Religions: And speaking of which, I've always wanted to see religions linked to culture. It avoids them being completely shut off from some players if they pick the wrong civ, while still keeping some degree of historical plausibility that, for instance, one needs Judaism, Israeli Culture and a Great Prophet to found Christianity. I'd also like to see greater mechanical differentiation between the religions, like in abbamouse's mod. Christianity spreading chiefly via missionaries, Judaism lacking missionaries but spreading naturally that much quicker, Hinduism being much more apt for converts among cities and civs with Indian culture, Islamic civs getting a penalty for violating halal etc. Should only be optional, of course, but I think it could work out as an intriguing, realistic, historically accurate and balanced solution to the Asatru Zulu problem.