Ahhh, Barbarian Invasion. Though I've only had it for two days, I think I can give a pretty-good review.
Barbarian Invasion is like revisiting M:TW for me, whereas Rome:Total War took out many aspects such as religion, multiple factions put in very small areas which meant expansion meant wars; small factions/big factions; Re-occuring factions in the forms of hordes; and better AI.
In more specific terms, BI returns to some very good ideas. Religion in R:TW was in my opinion, pretty useless, other than an occasional Bonus. Sure, you got extra experience and a little happiness, but then I often found no matter how high my Temples were in City level, cities would still face high squalor and other troublesome problems like revolts and disobedience. But in BI this is addressed through the general populace being religious, and thus more loyal if you are of the same religion. Cities tend to be more loyal, and though taking cities and converting them is sometimes a bit of a hassle, it's well worth it in the end, as you get a very loyal city which you can then tax the maximum limit.
Now, I've heard some complaints of "Not enough provinces". Personally I don't see this as a huge problem, as there are plenty of provinces, just not in the area that they used to be. Germania has tons of provinces now; where there might have been one in R:TW, there are now three. WHile the Roman Territories don't have as many, I find two reasons for such. One, it is easier to manage. Do you really want a whole bunch of provinces capable of revolting, and you having to take them all back? And secondly, a shift in historical power. Back in R:TW, multiple cities on the Roman peninsula were "powerhouse" cities, or otherwise important. But over the years power shifts, and some cities become more powerful than others, and thus may influence them more as subordinating cities rather than rivals. The opposite for Barbarium territories is true also. More cities sprang up over the years, and thusly became more powerful and rivaled others. Thusly, more provinces came up from spliting with other cities that might have once dominated over them when they were struggling villages.Other aspects. The Hordes are a great idea, but very frightening in some ways. I've noticed that sometimes one horde early on will take out another faction and force it to become a horde, but not take their city, thusly making two hordes to roam around. I've noticed on that same note that hordes take a while to settle. The Vandals seem to take forever to get to Gaul where they usually settle, and on the way they constantly seige cities and then run off, only to do it again. Annoying, but tolerable. Seemingly the best way to take down a horde is to kill the faction leaders and family.
And then there is the AI. Yes, improved indeed. In BI I have actually seen units of my own being routed, which almost never happened in R:TW. The AI is pretty tricky at times, but dealable with, and though it is nice to have improved AI, it really doesn't affect me.
And lastly, the faction layouts and diplomacy with them. I like how there are multiple Germanic Tribes up north all vying with each other, and how random it turns out. One campaign Burgundii are claiming multiple territories, and another it's the Saxons. The WRE and ERE aren't that big of threats, of course. The ERE are always too busy with the Persians and the WRE just falls apart most of the time, losing provinces and just withdrawing to their remaining ones. Smart move, but I haven't seen them strike out to regain ones taken. And is it just me, or do factions in this game seem overly-eager to get ceasefires and Alliances than in R:TW? Truly, one turn I could be at war with them, and by the next we're allies again. Sometimes annoying, but sometimes good when I need them off my back for a while.
Overall, 9.5/10.0 I would have liked to see Loyalty for Barbarian family members too, but sadly, nothing to be had.
And it's great to see Roman armies losing to barbarian AIs.
Nations to play in MIITW:
Holy Roman Empire
Poland