I loved the old block format. The one we see in Tempest, (Not so much but still kinda Urza's), Masques, Invasion, Odyssey, and Onslaught, as well as some later sets like Time Spiral and Scars. The large set would introduce a theme and mechanics, and then the small sets would evolve the dominant themes while adding twists that gave them their own identity. There's a definite progress in how the Rath Cycle uses its assets, with Buyback morphing gradually until you finally got nonmana executions in Exodus. All the while Slivers, Spikes, and Shadow rose, fell, and morphed to get a picture that FELT complete by the end of the third act. At the same time, sets also had identity: Stronghold had a strong "walls" theme and "Discard a card" costs were everywhere in Exodus. In Masques the rebels and mercenaries were very formulaic, but they shifted a little out of their roles for Nemesis (Which also had its own gimmick, Fading) and fully busted free with things like
Rebel Informer in Prophecy (personal Gimmick: Rhystic).
As much I hate Onslaught, even it had a decent progression of "Tribal" -- Legion was an "Extreme" gimmick (being the All-creature set) but Scourge was, at least mechanically, a good capstone to what they had been building. Mirrodin 1.0 though was the first time the theme never really evolved. It did artifacts and... yeah the second set had Indestructible and the third set did Sunburst, but they didn't have much to do with each other and didn't really have the feeling of progression that most blocks up until then did. Time Spiral, I think, is probably the block that really knocked out of the park what was great about the old three-set block format: It had a solid identity for each set, unified by a natural progression through a common theme, and during which the mechanics that would be used throughout were used in new ways in each set. The last time I feel it was really nailed was Scars of Mirrodin, in which the growth of the Phyrexian faction provided direction to the block.
I will mourn the passing of blocks, but in a way they'd been dead or dying since Lorwyn/Shadowmoor lost a lot of what had been good about them. When two-set blocks set in, we already weren't getting evolution, but rather a setup-and-swerve, and while sometimes the ability to have a lead in rather than just skipping to the payoff was used well (SoI, for all its flaws. Probably Amonkhet, really, but we'll see how that goes) other times it feels like you could have started with the second set shift and just been self-contained. How cool would it have been to get one big set where the Eldrazi are back and now they aren't just all colorless, they
actually require you to generate colorless mana rather than generic. Kaladesh was... well, I didn't much care for it or its story, but it easily could have been compressed to make the story of Aether Revolt the crux of the whole thing, there didn't really feel like there had to be a second set to move the timeline forward all those... days I think?... that it moved forward.
So, I'm cautiously optimistic about the change. I feel like they can do things without blocks that they couldn't do with it. And I really like that the Gatewatch is going to get de-emphasized, pretty hard by the sound of it. But at the same time, the key word is cautious. I'm actually kind of scared for magic in a way I've never been before, not because of this change and what this change entails, but because meta-structure has changed repeatedly in a rather short time. It feels kind of like Wizards of the Coast is flailing, and that does not instill me with joy nor confidence.