I'll break it down in numbers on how the 'smart loot' works:

Let's say, you're playing a Demon Hunter. Here is the breakdown of gears you will find:

• 60% of the gear you find will be general (all class equip-able) roll for your class (DEX + other essential stats)
• 20% of the gear you find will be class-specific (e.g. DH - 1h-xbow) roll for your class (DEX + other essential stats)
• 10% of the gear you find will roll for another class' stats, but it's an equip-able equip by your class (e.g. INT ring)
• 10% of the gear you find will be other class' specific (e.g. Monk's Daibo) that rolls for that specific class

In essence, 80% will only be gears you can use for that character, and 20% will allow you to equip other classes.