It's been a long time, but IIRC you could still group units in SC1 and bind them to number keys. So maybe only 12 at a time, but you could still navigate multiple groups of 12 with just a few keypresses.
But it still means attack-moving 108 units (aka: 9 groups, all labeled 1, 2, 3, 4, 5, 6, 7, 8, 9) requires 18-actions.
That is: 1 (select group 1), then a, then left click on the attack-area. Then 2 (select group 2), a, left click on attack area. Etc. etc.
That's a difference of 18-apm (Starcraft Brood War) vs 2-apm (Starcraft 2). Furthermore, that "Group 1" in SC2 could have 150-units or 200-units (or 400-zerglings, lol) in it. They pretty much made death-balls have 10% of the APM requirement in SC2 compared to SC:Brood War.
I'm all for making the game easier and more accessible: but the death-ball tactic was a little bit "too obvious" and kind of unfun to watch, compared to the tightly coordinated groups of 12 that SC: Brood War was famous for. The groups were too big, numerical advantage was the biggest advantage in SC 2 IMO.
SC2 having larger groups has allowed a different focus of micro though - stutter-stepping marines is a tactic that was out of reach of most players in SC1 but an absolute necessity in SC2 and the addition of unit abilities has decreased the utility of giganto deathballs - the micro is still very much a requirement.
Not to mention AoE attacks making splits important, as well as melee units still requiring careful setup of engagements for success. SC2 is extremely micro intensive, especially for how fast it is (in fact I dislike how heavily weighted it is towards success), and it feels a lot less like you're spending your APM fighting the game and your own units and more like you're helping your own units against your opponent's units.
SC:BW still had AoE attacks, arguably more important ones too.
Nuke and Psi Storm exist in both games, but Plague, Irradiate, Ensnare, Stasis Fields are all AoE effects... as well as AoE damage sources (Tanks, Archons, Valkyries, Devourers, and kinda-sorta Mutalisks).
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Lets put it this way: if you really wanted to, you could just have 12-units per group in SC2. But no one would do that, the APM-advantage of having a big group (and manually splitting in the rare cases where a split is needed) is far superior than using 9 groups to split up 108 units.
And sure, stronger players probably split their death-balls into smaller groupings of maybe 30 units or so (you don't want to just A-move all over the place, a degree of flexibility is useful), and only occassionally merges everyone together into a deathball. (Fast and loose is good: you don't want to dedicate an entire army to a battle, especially if they might lose).
But no one is making groups of ~12 in SC2. Its too small.
I don't see it mentioned here but the other limiting factor in Brood War is the pathfinding AI doesn't use modern techniques like flocking, so moving 100 zealots in the same direction is actually a lot of work if there are any obstacles along the path.
