Hooks are a good addition.
The culture/religion rework is appreciated, I like that it makes some contentious areas such as the Iberian taifat or Bosnia more difficult to hold / unproductive for the usual empires. At least in expectation, as I didn't play long enough to judge. Also the ditching of heresies as "derivative" of another religion, with particularly egregious cases such as Manichaean being spun off Zoroastrianism, or Yazidi or Druze off Islam.
Prestige matters more and piety is harder to come by... until you get a decent income, at which point it becomes more trivial to attain than in CK2 (pilgrimages are too strong). Not good.
No more popup spam from calling 50 tribal vassal allies to war. No more ship management. The bland CK2 tech system and sending your spymaster to Constantinople every game is gone.
Game is less cluttered with marginal / pointless "flavour" than CK2 (this will change, because it's so easy to get suckered into accepting DLC as healthy and normal, I made this mistake with CK2 myself -- never again! -- and CK3 will run the exact damn same gauntlet).
That's all the good points.
The user interface is abysmal. It's unresponsive, it hides its information behind multiple layers of tooltips, it has hover-menus you have to fix via right-click before you can use them productively, it's cluttered all over the place, it seems designed for mobile devices. Event boxes take up ridiculous space to accommodate the unnecessary ragdolls -- add a scheme notification above, a battle overview below, charinfo to the left, council to the right, and every single UI element gets into the way of another one. Mapmodes have been cut, with some of the most useful from CK2 (e.g. Show Vassal Realms) or even simple political (!) being gone, after simple terrain has already been ditched ever since PDX went over to map pornography.
Barony-tier holdings as atomic provinces add nothing, no tactical depth (there are not nearly enough combat units on the map for that to come into play, this isn't Hearts of Iron), no strategic granularity, not even anything LARP-wise. It just leads to more clicking as you chase armies. They might as well still be in the counties.
MTTH is dead, long live the progress gauge. This is a change almost across-the-board, everything is a gauge now. I like this somewhat in the case of stress (makes more sense than a binary switch), except that stress becomes too easy to manage with merely decent income (again). For everything else, e.g. schemes, cultural conversion, fabricating claims, it's a step back to making the game even easier to control without any possible interference from non-player agents (and be it just Fortune). Cultural conversion is especially ridiculous at the scale it allows; I played as Gudrød Crovan of the Isles and assimilated a third of Scotland, as well as Ulster, to Turbo-Norwegian by like 1100.
Expansion is easier than ever, yet I can't press claims for two counties in the same war. Keeping your realm united (or reuniting it quickly) is easier than ever, yet I cannot switch out of gavelkind until the 13th century due to technology being "communal" / "cultural" now, which replaced a bad system with a worse one.
Navies being gone is one of the worst changes. The computer embarks for no reason, disembarks when your armies are close, tends to sail straight for your capital in any defensive war (granted, this is true for CK2 as well, but that game is 8 years older), and my alliance with Duke Ota of Bohemia regularly resulted in 2000 troops fighting by my side on the British Isles, instead of any sensible local diplomacy constraining my expansion.
The wargame side became even poorer. Flanks are gone. Tactics also seem to be gone. Cultural retinues likewise. The new retinues are called men-at-arms. The computer only ever seems to use light infantry and cavalry men-at-arms, which means you should build archers and pikes only and bolster them with duchy buildings, creating super-soldiers by 1100. It took an alliance of Munster, Gwynedd, Scotland and England to finally force me to cede one county at one point, while I was also fighting Norway.
Indeed, wars are generally decided by alliances before they even start, not strategy or tactics. Since these largely rest on marrying your children to the right people and/or constantly using Sway schemes with a Diplomatic skill tree perk to enable alliances without familial relations, they are also not very interesting.
Tyranny is much less important because you also accumulate dread, which also pacifies vassals.
On the Believable Worlds front, Lappland constantly invaded Scotland (and unsuccessfully England), Scandinavians still make the Baltic coast a patchwork for little reason and obliterate most of the pagans long before the 13th century, Jerusalem flipped to Islam shortly after the First Crusade (lol), the HRE abandons Italy if it rebels even once and still habitually consumes France early, and most egregiously, BYZ is stable beyond belief and grew to Ottomans-at-their-zenith proportions in my game -- again, by ~1100. Seljuks and Fatimids didn't even appear to have clashed at all. Poland snakes into Russia while having lost most of Volhynia. And so on. Most of these behaviours are either the same as in CK2, or downgrades. I really didn't notice any region that behaved more plausibly -- maybe India, which seemed unusually stable rather than housing 3 empires.
Speaking of which, the fantasy empires such as "Carpathia", "Britannia", "Scandinavia", "the Wendish Empire" etc. still exist (and "Maghreb" is imho a terrible new addition for the sake of "balance" only), which again just makes managing big realms easier yet since you can delegate to de-jure vassal kings, and gavelkind won't split the de-jure empire. There should be just five de-jure empires: HRE, ERE, Arabia (to represent the Rashidun Caliphate), Persia and Tibet. The "Charlemagne button" for custom realms is fine since it requires extraordinary prestige; but otherwise, claiming unprecedented Imperial authority in e.g. Scandinavia should not be as easy as the restoration of a traditional empire.
I realize PDX have long since ceased making strategy games or "historical simulators" anyhow. See also Stellaris. See also HOI4. See also CK1/2, really.
I hope for "indie" devs to rise to influence and take a good chunk out of pernicious neo-PDX's wholly undeserved near-monopoly. I've said this elsewhere, but Rhye's and Fall of Civilization is a better strategy game and history simulator than any PDX game after EU2.