Why does everybody seem to want to slowly add more and more medieval stuff to the game? Revived Roman Empire? Byzantine cores on central Turkey? Releasable Al-Andalus? A formable Caliphate? Surely these things belong in CK, not in EU, which is about the Renaissance and 'enlightenment' periods. The shift back to 1399 as opposed to 1453 was bad enough, but can be coped with. But EUIII doesn't simulate medieval politics at all, and pushing the game further back just makes things more and more unrealistic.
I enjoy a good Byzantium game, but maybe that's just me. I also enjoy the start-up scenario for the variety of states available; after all, there are only so many times you can play as England, France, Spain, Portugal, or the Netherlands before the game gets boring. I find it interesting to play as deceased states or minor cultures like Balkan nations, Greece/Byzantium, and other people that were absorbed and assimilated by the rising powers during this era.
Plus, most people play the start date which is, in fact, part of the Middle Ages.
EUIII doesn't end in 1836, and EU:R doesn't end in 1000 AD. There is always a bit of a gap between the games. I can cope with 1399, since that's basically the very late middle ages, but adding lots of medieval mechanics just intrudes on the rest of the game. Look at all the effort put into Byzantium, for example. That time could easily have gone into more cool stuff for Poland-Lithuania, but it was instead used to create a set of fantasy missions for a dead Empire with only symbolic importance. Then there are all the releasable French minors, constant holy wars and a strange 'crusade' system.
What? 1399 wasn't the "very late" Middle Ages. The Middle Ages didn't really end until the fall of Constantinople, the end of Burgundy, the discovery of America, or even the start of the Reformation.
That is a good fourth of the game and the only fourth that most people play. A previous poll held by Johan shows that most players start in 1399 almost exclusively, and if you look at the Post your Empires thread, you'd see that most people stop playing around 1550/1600. Personally, I've never played a game starting in 1399 past 1700 either.
That said, I think you'd be highly displeased to know that most major mods set the start date back to 1356.