I love the Savage Frontier sourcebook and the 1st Forgotten Realms Campaign set. I thought The North sourcebook nicely expanded but i sincerely don't recall the differences between them that much, only that it used it as base and expanded it significantly.
Reading the Savage Frontier inspired me to launch a campaign in the North back then. Awesome cover art too! I also had the D&D card of that half-orc warlord who's name i can't remember now. Uthgarth or something IIRC.
I'm currently working on an idea using only the Campaign Set and Savage Frontier. Going through both of them with a fine comb keeps digging up lots of new interesting discoveries. The history of the region is kept really quite vague, but the detailed timeline of historic events that was established later contradicts it at several points. And it's overall much more basic.
The North has exactly two ancient elven Kingdoms: Illefarn and Eaerlann, and that's it. Netheril had wizards, but no mention of flying cities, and it declined over many centuries and not in one sudden event. The people of Silverymoon, Sundabar, and Everlund are the remnants of the Netherese and not related to the Northmen or the Southerners at all. Silverymoon apparently has a few dozen elves living or visiting there, not 10,000, and is considerably smaller than Sundabar. The dwarves are down to only two strongholds, which couldn't be more further apart on the edge of the Anauroch and the coast of Icewind Dale. And as I mentioned, the area is double the size of what it is in later editions.
I find this all extremely fascinating, and a great place for a fresh start to run a campaign that reimagines what the region and the Forgotten Realms as a whole can be. Not being much of an expert on Greyhawk, but it all feels quite similar to the presentation I've seen in various Greyhawk-based modules.
Though at the same time, The Savage Frontier doesn't actually provide a great deal to work with. My first idea was to start a campaign in Secomber, with a party of adventurers just arriving from the South to gain their first foothold in the area. But there's not really anything to give any kind of exciting look and feel to the lower Delimbiyr Valley. What do you do there?
Even if you assume that the region has only been resettled by people from Waterdeep in the last two or three hundred years (implied by several references), and that Secomber and Loudwater are build on the ruins of much older post-Netherese towns, what does the place feel like? What's going on in the area? What are the threats? Of course it's easy enough to make something up from scratch. But that's something you can do anywhere else just as well. How would it be different from some remote villages outside Baldur's Gate, in Sembia, or Amn? The Dessarin Valley is similarly unexciting. Things look more interesting further north near the Spine of the World, where there's a lot more stuff going on. Abandoned dwarf strongholds. Orc tribes. Uthgardt barbarians. Trolls. Undead. Foreboding Mountains. I can absolutely understand why in 3rd edition, they only updated the Silver Marches instead of the whole North.
I normally would say that the region has a lot of fat to trim, even all the way back at the start. Except that there isn't even any fat. Just emptiness.