Heavy pretty much slows down your physical movement speed. Especially pesky when you get slapped with it right before an AoE and cannot escape in time.

Cross Class skills for classes work slightly different than they do for jobs. Classes can equip any cross-class'able skill from any other class. At max level (50), they can equip 10 of them. Even if they make no sense - like a Mage with Provoke. Jobs get clipped to only having 5 slots for cross class skills, and they can only pull cross-class'able skills from two very specific classes depending on what their job is.

Example: Pugilist can pull skills from any other class. Monks can only pull skills from Marauder and Lancer. Arcanist can pull any they like, but Summoners can only pull from Archers and Thaumaturges. etc etc.