In my opinion as a Zerg player you do not have to rely as strong as the other races on microing. Most units dont have special abilties or buttons one can press (yeah every unit can burrow ^^) but you dont have to use abilites like blink, stim packs, forcefields, emp ... and and There is only one unit that is considered a caster and its rarely used by most of the players. I think the Zerg are very makro, heavy you need more expos then with other races, you can pump out an enormerous amount of units very fast. You can also hotkey all your hives/lairs .. to one single key and produce very fast and very efficient instead of hotkey numerous buildings or buildinggroups, its more like C&c and having an easy accecible production tab instead of having to use single buildings. The Protoss for me, are somewhat the opposite and they require a very micro intesive playstyle, Terrans are somewhat in between.
I wanted to say that Zerg needs micro too because of the limitation on the number of units in a group but I don't know how the increased number from SC changed that, so it might not be the case. But I'm assuming that's the reason for few active Zerg abilities.
Zergs require a lot of micro in my own opinion because if you don't then your forces can easily get slaughtered especially the Zerglings. Roaches for one can burrow and go behind your opponent and surprise attack them. Then again that's tactics not necessarily micro unless your doing it all in one motion along with the rest of your attacking/defending forces.
Infestor use requires a lot of micro for efficient use, and it is invaluable as opponents start to mass MMM or bring in Colossi or Thors.
I am not talking about the standart micro that everyone has to do (positioning, wounded units, etc etc.) If you take away what everybody else has to do in terms of micro, there is not much left compared to the other races.
They require more of what you are referring to as "standard micro" due to their greater fragility and, in the case of lings/banelings, lack of range.
