Brood War is much more dependent on micro than Starcraft 2. Sean "Day[9]" Plott has mentioned this a lot in his videos. You can only select 12 units at a time and cannot select more than one building in Brood War. And there is a bit of latency in Brood War that doesn't exist in SC2.
One such video (skip to about 4:30 for opinions on mechanics): https://www.youtube.com/watch?v=EP9F-AZezCU