Why is JaeDong's Mutalisk Micro hard/impossible to do in Starcraft 2?
In general, 'counters' in Brood War are much less definitive than they are in SC2, and there's no exception here. In StarCraft 1, Mutalisks are not very fast and have a smaller range. Stimmed Marines can nearly catch them, but they only have 40 health and start with 4 range instead of 5. In Brood War, Mutalisks can actually engage Marine groups and kill the outside ones without taking enormous amounts of damage. When repeating this, it's possible to open up an opponent with your harass and force him to spread units so that you can take advantage of weak points in his defense. However, few maps in have areas near bases where you can hide your Mutalisks risk-free. Your units can thus be cut off by Marines much more easily and must be constantly looked after. Turrets are also cheaper, but only deal half damage (10 of 20, instead of SC2's flat 24) to Mutalisks and so can be fought around more easily. Again, Brood War has soft counters, not hard counters.
Mutalisk micro in StarCraft 1 is made versatile because of the mechanics of the game:
- StarCraft 1 is played professionally with a latency of only a few milliseconds. SC2 is played with a minimum latency of ~200ms, making controlling units with exacting precision essentially impossible.
- When units are given a command, they will move in a formation if all of the selected units are within what's called a 'magic box' (the original use for the term). If any single selected unit is outside of this box, the units will each try to execute the same command. In the case of flying air units like Mutalisks, this means that they will stack on top of each other and can effectively be microed like a single unit. The typical way to accomplish this is by selecting 11 Mutalisks in addition to a pinned-in Zergling, an overlord, or a larva.
- Because pathing isn't very good, ground units are occasionally out of place and can be eliminated. In SC2, units are frequently just clumped into an impenetrable ball.
- A final mechanic that leads to effective Mutalisk harass is that in Brood War, air units that attack from high ground don't reveal themselves and thus can't be attacked. Because of this, Mutalisks could be microed from fog of war to deal huge damage. This was changed in SC2 for no apparent reason, making tactical maneuvering of Mutalisks unimpressively dumbed down.
Jaedong was the first to micro two control groups of Mutalisks at the first time effectively while still macroing well. This (incredibly difficult) technique allowed him to do things no other player was capable of and unlike most players who built an initial wave of Mutalisks and used them to deal as much damage as possible until they died, Jaedong often strengthened his air army as the game went on and won with it.
Here's an example game vs. Iris, a consistently strong Terran. Two-group micro starts a few minutes later, at 11:30. Jaedong is engaging and frequently targeting specific out of place units. After the attack projectile flies, he immediately pulls back to minimize damage. How well he does this determines how much damage he takes. As you can see, once Jaedong enters the base he is CONSTANTLY forced to micro his Mutalisks until he departs, and departing causes him to take exit damage. There's no corner or high ground area for him to sit in while he macros, he has to do it on the fly.
After typing all that I realize that he also might be talking about Zerg vs. Zerg, which Jaedong at one point had an absurd 80% career winrate in. The matchup is frequently referred to as JvZ. Nearly every game in this matchup in SC:BW became a micro heavy battle between Zerglings, Mutalisks and Scourge for each player. Here's a standard midgame of ZvZ. Mutalisks that are microed well can kill Scourge (flying single-target suicide bombs) but it's ridiculously hard to do, especially during a battle. Jaedong does it easily, like this game, and routinely wins because of it.
This is just to supplement Decency's answer (below) for readers who are unfamiliar with Brood War; please read it first https://gaming.stackexchange.com/a/74844/20893. I agree with him, you just can't do BW muta micro in SC2. But what exactly is it that you can't do?
Mutalist Micro
Sometimes it's not clear what BW players mean by "mutalisk micro". I think it's easier to show in-action than to describe it in words, but I'll try both.
Along the veins of the Jaedong vs UpMagic video in the question, and the Jaedong vs Iris video in Decency's answer, I'll add in Jaedong vs Fantasy (it's actually not his best) and July vs Hwasing (how it all started).
The Liquipedia article on Mutalisk Harassment is a textbook resource explaining the mechanics and advantages of muta micro, but it doesn't hurt to repeat the points here.
Grouping
Allows the mutalisks to be moved like 1 unit
Allows mutalisks to stack their damage on the same target
Prevents opponents from focus firing down single mutalisks. The targetting AI would generally pick a different mutalisk every time the mutas moved out and then back into range. Since the mutas were stacked on top of each other, an opponent had a hard time clicking and picking a specific mutalisk
Targetting & Moving
Mutalisks belonged to a group of units (Vultures, Wraiths) that did not need to decelerate while firing because they fired projectiles. They could fire on the move!
As long as the the target was < ~30° of the direction the mutas were facing, the projectile would fire. This meant that sometimes the mutas looked like they could shoot sideways or backwards.
Slight variations in how the game interpreted the Patrol, Hold Position and Attack-Target commands meant the mutas could concentrate all their fire on one target or spread out the damage amongst multiple targets to prevent overkilling one unit.
Here are some detailed discussions on how to pull it off.
Result
At the risk of sensationalizing the results of mutalisk micro (because it's not quite this good), imagine fielding a 1320 HP (120 HP x 11 mutas) unit that dealt 99 dmg to the first target (no upgrades on either side), and then 33 dmg to the second target and 11 dmg to the third target. Then imagine that it didn't even need to pause to attack, could shoot sideways, could turn 180° instantaneously, and slowly regenerated health. O_o!?
In the case of Jaedong, because he could micro 2 control groups of mutalisks (it's mind boggling to think about the mechanics of it), his Mutabehemoth was a 2640 HP, 198 dmg (then 66, then 22) natural disaster, rampaging across the map.
You'll notice in the Jaedong vs any-other-player games, the commentators' camera almost never leave the mutas. That's where the game is - the whole game. Jaedong's mutalisk micro controlled the game.
Now imagine taking that away from him.
The Pain of Switching Over to SC2
In the case of Jaedong, his virtuoso mechanics depended on the idiosyncrasies of BW. I really appreciate that Decency explains the latency reason for why it can't be done.
Even for the average competitive player, the exploits in BW made the game distinctive. For the more dedicated Zerg player, deep understandings of topics such as "Which Way Does the Mutalisk Glave Wurm Attack?" meant that SC2 feels like a completely different game.