I'm playing with fire and sword right now, and even with the latest patch it's really buggy. Nothing game breaking but still. It has a lot of new options, but it's mostly those new options that weren't polished.
For example, during the siege, you now have multiple ladders, but the problem is, the defendant’s army AI still thinks there is only one ladder. So when the battle starts, my men cluster on one ladder, leaving the other completely unmanned, which sucks. Big time.
Also there are was a lot of balancing done. The pikeman have huge range with pikes and are better in stopping cavalry in their charge (fakedit: actually, this may be the same as warband), the foot soldiers usually have 2h weapons which means a lot because you can no longer use 2h weapons while mounted. That's right, no more wielding huge ass cleavers, claymores and polearms (lances are available while mounted) and cutting men without even getting in their range.
Also firearms.. You get hit, you die. Right now I have (probably) the best armor in the game, and can still take only a couple of bullets before I die. Headshot is instant kill. Also your horse dies much quicker as well cause of this, often leaving you in the midst of the enemy with only 1 point in athletics -.-
So basically, you are no longer the one man army, the jedi on the battleground. You can no longer charge into enemy ranks and reduce their numbers before sending your army in. You can no longer pull out epic wins, and if you do, it was just luck for not getting shot.
Some may like it cause it IS more realistic, and more balanced, but it's just not that fun to me.
There is an actual storyline, which I am following. It gives you some purpose instead of just roaming around endlessly, which makes it easier to stay entertained.
Also many other things, but this is the gist of it.
EDIT: very few mods compared to warband