Originally Posted by Sarvius

Like Jasos hinted at, the only thing that stalls macros these days is when you have multiple lines of commands and more than one of them would trigger a gcd.

So pre-patch, you could not have had the following macro for example:

/cast Adrenaline Rush
/cast Sinister Strike
/startattack

The macro would successfully cast adrenaline rush the first time and then would simply return an ability-not-ready error on every consecutive keypress. You can, however, have macros with multiple abilities as long as only one of them invokes a gcd. So it's fine to have /cast Premeditation /cast Ambush or /cast Shadowstep /cast Vanish.