To me the most notable change was to nagash. On the positive side, he's now available to every death faction. On the negative side, this new version is ~heavily~ nerfed.
1) his command ability has been completely rewritten. It now grants +1 to another death unit's ward save for a single combat phase. Note it only lasts for a single combat phase and he can't use it on himself. This isn't a terrible or useless ability by any stretch, but compared to his previous ability that granted your entire army (including himself) reroll 1's on saves and attacks and immunity to battleshock from your hero phase until your next hero phase - ie up from your first hero phase until nagash dies. This is a massive nerf for summonable heavy soulblight nagash armies which depended heavily on the battleshock immunity to make up for how much nagash cut into your numbers. This is a massive nerf to nagash himself regardless of what army you take with him with because there's a vast difference between a 2+ save and a 2+ save re-rolling ones. Granted a 2+ rerollable is obnoxious and probably shouldn't exist regardless, but good for the game or not its much weaker than it was.
2) nagash cannot benefit from allegiance abilities. Not a huge deal in soulblight as most of their traits were written in a way that he didn't benefit anyway, but a major blow in OBR where he can no longer benefit from petrifex's protection from rend. He also probably can't make use of emerald host bodyguard hexwraiths in nighhaunt, though that's still contentious. He doesn't even get the usual 6++ death save, so Nagash has no ward at all against normal damage.
3) in OBR nagash also cannot heal himself with his invocation anymore.
4) And he's still restricted by the relentless discipline rule so he can't use normal command points and generic command abilities, even though he now also can neither use nor benefit from OBR faction specific command abilities. He can use RDP to power his own command ability but otherwise can't use or benefit from CAs at all in an OBR army. Someone /really/ didn't want nagash to run in obr anymore.
5) damage output reduced - his spirit hosts hit and wound a bit better but no longer deal mortals, likely a preview of what spirit host attacks will look like whenever nighthaunt get their book. His sword now wounds on a 4+ instead of a 3+, which thematically confuses me (i could see him heeding a 4 to hit if they thought it needed a nerf, he's more wizard than warrior after all, but if he does hit shouldn't the god of death's own sword be overwhelmingly likely to wound?), but whatev.. These aren't major nerfs, but they do combine with the above to further weaken our patron deity.
On a more positive note, he does get full access to the spell lores of whatever death faction you run him with. Still, despite all the nerfs his points cost is still 955. Either he was woefully underpriced before - which I highly doubt - or new nagash is easily 100 points or more overpriced. IMO you could drop this version of Nagash back down to 800 points without causing problems.
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It's not all down sides - he does now enhance abilities that restore slain undead throughout whatever army he's in, though it's awkwardly worded and basically requires further clarification for how it interacts with basically every ability that restores slain undead, even his own. His invocation can return slain death models to a unit with a combined wounds total of no more than 3, while his supreme lord ability says that abilities you use to return slain death models can either re-roll the die to determine how many models are returned or can return one extra model. But invocation doesn't return a set number of models, it returns any number of OBR or Summonable Death models with a total wounds characteristic of up to three. So do you restore 'any number of models plus one, but they still must have a total wounds characteristic of no more than three since nothing about the ability mentions any exception to wound characteristic limits' or do you restore 'any number of models with a combined wound characteristic of up to 3+1=4' or do you restore 'any number of models with a comined wound characteristic of up to 3, plus one extra model total regardless of wound characteristic? And if it's the latter, does the ability work even if the total number of models you were initially able to return to the unit was zero? Like if I field a unit of 6 wound morghasts and one of them dies, can nagash's invocation restore 0+1=1 of them?
Or what about the Gothizzar harvester. If a model near the gothizzar harvester dies, then on a 4+ the harvester can return slain models to a nearby OBR unit with a combined wounds characteristic limit based on the model that was slain. Does nagash's ability reroll the 4+, or return one additional model but you're still limited by the same max wounds characteristic maximum, or return any number of models up to the given wound characteristic maximum plus one additional model regardless of wounds characteristic? In practice, do you get a re-roll on the 4+, or do the abilities not interact at all, or, if using the ability as is typical to use slain mortek guard to restore mortek guard to that same unit, do you roll a die for each mortek guard slain in range and restore /two/ slain mortek guard for each 4+?
Or what about Deathrattle Skeletons? Before they activate in combat you roll a die for each skeleton slain earlier in that same combat phase, and on a 4+ you return that slain model to the unit. So do you get rerolls on all those rolls? Or just one of them? Or do you not reroll the die because it's for triggering the ability not setting the number returned, and instead for each 4+ you return up to two slain skeletons? Or do you roll all your dice, return a skeleton for each 4+ rolled, and then restore just one additional skeleton total? Do the additional skeletons (if any?) provided by nagash's ability also have to have been slain earlier in that same combat, or can they have been slain at some previous point in the game?
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So... what armies will want to run this new Nagash? As expensive as he is, I don't know.
Maybe Nighthaunt? They're so desperate for monstrous heroes, especially ones that can take a hit - even if his durability isn't anything like what his points suggest, he's still probably not going down in a single round the way literally any other nighthaunt hero will should the opponent choose to focus on them. And they've got some ok spells for him. People will certainly try.
For FEC... If he had kept the battleshock immunity CA I could maybe see some sort of fluffy ghoul horde but as is no. Honestly even then, FEC are exactly the opposite of Nighthaunt when it comes to access to multiple strong, points efficient monstrous heroes. I have a hard time envisioning a FEC list with Nagash that isn't going to do a lot more using those points on a pair of Terrorgheist Kings.
For Soulblight and OBR, I don't know. I guess it depends in part on how the supreme lord ability ends up interacting or not interacting with their various recursion abilities?
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IMO 955 is just too much for this version of nagash, regardless of the list you put him in, but honestly I'm not too upset about it. Being able to run him in Nighthaunt and FEC at all is fantastic! And if you ignore the points cost, his warscroll is still plenty impressive, still conveying that 'god of undeath' feel. If I'm right and the points need to come down, then tournament usage and results will show that and over time it'll happen. In the mean time, I've always felt Nagash was a better fit conceptually for narrative big games rather than the 2k competitive tournament scene, so if that's where Nagash lives for a while I'll be ok with it.
Then again, I wasn't planning on taking him to any events this year.
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I've just updated and I have given the Black Knights 3 out of 5 now. They might even be stronger than that, only playtests will tell. 
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