Juicy Realm Crack

The Great Houses
There are a lot of differences in how the Great Houses are portrayed in each edition. 1e goes into a bit of history and why they have power; 2e focuses almost exclusively on why the Houses want the throne (and has a separate section about likely contenders from each house) or form their own kingdom, and 3e, in its short section, tends to focus on how the Houses interact with the world and get their power.
I'm probably going to miss a lot of details, but a few things that struck me:
House Mnemon really became one dimensional in 2e. In the other editions, it gets its power through its close ties to the Immaculate Order and political skill. 1e says Mnemon, if she took the throne, would crack down on the Hundred Gods Heresy. 2e says that much of their power lies in sorcery, Mnemon is a great sorcerer and furious that she couldn't maintain the throne if she took it, as other Houses would team up against her. I get the impression of a ranting supervillain. She's also described as the Empress' only non-senile daughter, which isn't the case in 1e, where V'Neef is led by the Empress' youngest daughter.
1e notes that Ragara, one of the Empress' oldest sons and founders of the House, is still alive, though in ill health, and he doesn't live with most of the family.
Peleps underwent a huge change from 1e to 2e. Originally they were not only in charge of the Imperial Navy, but had a lot of power through forming coalitions in the Deliberative and a strong control of the Realm's judges. In 2e, they seem more interested in forming their own mini-nation of islands, and people in the Threshold regard them as pirates. 3e says that their power makes other Houses alternate between stymying them and allying.
1e Sesus are portrayed as atheists and hedonists, but ironically they have a large number of DBs due to careful breeding. In 2e, they want to control the southeast part of the Blessed Isle, and their troops raid the land for supplies. In 3e, they are perceived as 'brutes and thugs.'
V'Neef, the youngest House, has also undergone some changes. In addition to its vineyards and merchant fleet, in 1e it gained part of its power from the right to hunt pirate ships, and its ties with the Immaculate Order. 2e has them in trouble, not sure if they want the throne or not, and losing territories because of it. It also says V'Neef Bijar thinks she might be able to get into the Imperial Manse, which is sort of a 'take the throne free' thing. (I also wonder where the name V'Neef came from; whatever imaginary languages produced the others seem to have different phonetic rules.)
In 1e, House Ledaal has a contingent of Sidereal advisors They're well aware of threats to the Realm other houses are ignoring or minimizing, such as the Deathlords. They also really don't want the throne, so much so that the other Houses assume that the stance is some sort of ploy. 2e, they want to maintain control of their part of the Blessed Isles, and will fight whoever takes the throne if that group doesn't support them. They've also advanced a potential Emperor, Kas, but he's considered a major sexual pervert.
House Nellens hasn't changed too much, though 2e seems to portray them as pretty dishonest. I like the detail in 1e that nobody knows why the Empress made them a major House, when they aren't her descendants, just distant relatives of one of her consorts.
House Tepet has been devastated in a military campaign, though the details of the campaign change from editions. In 2e, the Bull of the North defeated them. Probably their weakened position is why the Regent was chosen from their household; nobody thought there was a danger of the Tepets keeping control of the throne if Fokuf proved competent. It may also be why Roseblack, a Tepet, is on the shortlist for Empress; people think they can control her.
In 2e and 3e, Iselsi is officially dismembered and disgraced, though they continue to serve as intelligence agents. In 1e, it's a bit more nuanced. They haven't been dismembered, but they are exiled, taking advantage of the Immaculate Order's rules on sactuary to set themselves up in the Palace Divine, despite not being that devout.