Here are my thoughts on the horde phase. I actually like the way it is. I don't mind being able to conquer half the world... at least initially. The counter to that is obviously the fact that you are cut back to a single county after civilizing. That leaves a good chunck of the world as your religion and culture to now evolve on it's own. Here is what I think could be changed/improved.
1. I shouldn't retain strong claims on everything. It was too easy simply to "press all claims" and regain every bit of my lost territory within ten years of civilizing (all of africa and a good chunk of spain). I like to have to interact with other nations in other ways in order to claim land. That might include having to find those with claims on titles and get them into my court or various other intrigue options and using marriage. Right now I just declare war after war and there is no need to interact with any other nation in this way, including those of my religion and culture. In short, it makes it too easy to conquer half the world.
2. Even if I lost all my claims there are still a huge number of barbarians about that I can simply come in and basically get a free county from without even having any recently conquered penalties. I learned very quickly in my games to ignore civilized people in the beginning and go straight for barbarians. After the split up of my lands they were easy targets and I was able to accumulate a very large amount of land in a short time that included the vast majority of the land I gave up when I civilized. If perhaps there were a few more civilized peoples close to me, this fast early expansion could be prevented and if I conquered the civilized guys next to me via conquest I wouldn't be able to immediately pull up a larger levy to go hunting more like I could with the barbarians.
3. Also, it might help if barbarian lands took the recently conquered penalties as well when I capture them. It would help to mitigate the increase in the size of my army and allow some others time to expand some as well.
4. I don't like how when I have just crushed someone's army, and they are now retreating, they are able to conquer my land and recruit more soldiers the moment they arrive. I don't see these guys, who are battered and bloody and running for their lives, as having the time or being able to to totally pillage the land and force the people there to fight for them when those people know their true liege just kicked these guys butts and is on the way to do some more butt kicking. I think if the garrisons had maybe 10 or 20 people in them that this would be prevented from happening and provinces would still be very easy to capture. It would also prevent instances of a single soldier capturing a whole empire and winning the war single handedly while there is a 20k man army chasing him

(as Gladdig knows, I did this)
The small garrisons could be thought of as the women, children, the elderly, and those few left behind to protect them who are not off rampaging with the horde. They could put up atleast some resistance towards an army that is disorganized and in full retreat, running for their lives. Atleast for a day or two..long enough for the main army to arrive and drive them out again or kill off the rest.
All in all, I like the new system and look forward to experiencing some of the other events that happen later on.