Hi Rej_Draeger, I'm in a similar position to you in that I'm a GalCiv II fan without much time for playing lots of games; I also came to FE without having played or even really having heard of War of Magic. I pre-ordered during the Beta after being impressed by what I saw on the forums.
Firstly, if you enjoyed Twilight of the Arnor then Fallen Enchantress feels more "finished" than TA did on release. I'm thinking mainly faction differentiation here - TA had problems with the custom tech trees that were still being fixed (by modders) when I came back to it last year. By contrast, nothing in FE's 8 default factions feels that unfinished and all are competitive as player or AI sides, and even the two removed-but-not-really premade factions are fine.
The AI opponents are in a similar state - they're capable of enormous idiocy at times, and have a few annoying cheats available to them which I assume are unintended and will be fixed, but generally they have the cunning & capacity to surprise (disclosure: I'm not the best player in the world, so YMMV) that the GalCiv opponents had, without the sometimes game-spoiling problems that a couple of the AI personalities in TA exhibited. No-one fails to expand, or develop cities, for example, at least not on the settings I play. Where it falls down a bit is warfighting - in a recent game, for example, Yithril made a perfectly-timed declaration of war against me when I was over-stretched, fighting two other wars on opposite sides of the map, and most of my empire was divided from my capital and main fortress by his territory. However, he then conspicuously failed to exploit this, failing to capture several undefended cities until I could move defenders into position - by the time he took one of my cities, I had captured three of his and was beginning to steamroll him. I'm not too bothered as I have faith in Frogboy's AI coding, and this was only playing on Normal, but it's an example of how there's more work to be done. On the other hand, I have been outwitted a couple of times by AI players I thought weren't a threat.
I learned the basics of the game over about three test sandbox games during the Beta and my biggest downfall was failing to see that the sovereign settled the first city himself (I'm not sure what I expected to happen, I just got swept up in going around killing monsters and thought I would obtain a city or a settler or something sooner or later. Then I died). Once I got to grips with founding the first city, getting research going and TURNING OFF AUTO-TURN everything went remarkably smoothly. This was without the benefit of a manual or a worthwhile tutorial, so I expect it will be pretty easy to get going now. The forums and user guides are full of useful tips and worth a look, but are not essential reading in the sense that the game is unplayable without them. Same for mods - they're well worth looking into, but not required.
Your biggest requirement seems to be replayability, though. As far as I'm concerned, they pretty much have that nailed - at any rate I keep coming back to it. I haven't yet had one of the massive, epic games that GalCiv was famous for, but then again, I only actually had four or five of those playing GalCiv. The next level of enjoyment down - fun, tense games that you come away from feeling that you spent your time well and now you want to try playing this race or those settings - that's definitely there.