Heros
Heros are similar to planeswalkers, except they are not born with a spark, but given one by being kind. Some heros are planeswalkers, but most simply want to jump into the fight.
You may only have one hero in your deck, but you can have multiple copies of the same hero. You may only control one hero at a time, though multiple players can control the same hero. If you are playing a format with a commander, you may have any hero be your commander, but you cannot put a hero in your main deck.

New mechanics
Explosive - Explosive is a keyword mechanic. It means that the creature deals damage equal to its power to each creature blocked by it or that it blocks, rather than splitting it
Attacker - Attacker is a keyword mechanic meaning that the creature can't block. If a creature somehow gains attacker and defender, it can't attack or block. Attackers don't have to attack every combat, however. Attacker is meant to streamline some cards by reducing text on them.
Snipe - Snipe is a keyword mechanic that means you choose the blockers for the creature. You choose blockers after your opponent or opponents, and you can't choose illegal blocks. You have to choose at least one blocker if able, however.
True Sight - True Sight means that a creature can block and target creatures and permanents with abilitys that would normally prevent it from being blocked or targeted (such as hexproof)