As for melee game, most of the time you will get 2 heroes, which means you have to KNOW WHEN TO CAST WHAT FOR 2 HEROES (as compared to DOTA 1 HERO).

As for spell casting units , you will need to know when to cast what spell (from heal, inner fire, blood lust, cripple, unholy frenzy... too many spell to list down).
To make it simple, each of sorceress, witch doctor, necromancer, druid of wind ... etc.. has 3 skills to use.
As for other units, some of them also have skills, like dragon hawk rider, footman etc...
And you get to control like 10 to 20 of them during a battle...