6e has 32 new rules, including some things as basic as "d20 test," "occupied space," "object," "per day," and "passive perception." Interestingly:
- d20 Test seems to finally put a label on the source of ability checks, attack rolls, and saving throws. For those who knew they were all basically the same roll, couldn't quite put a finger on it, and wanted to call it something other than "check" anyway.
- An occupied space has a creature in it or is completely filled by objects. Therefore, a section of stone wall, 5x5x5 feet, that has suffered the loss of one stone, is not occupied.
- Undead are objects: as non-living, distinct things. If they weren't before.
- Days are not defined by hours. They're defined by rests. The sun will be very confused.
- Passive perception seems to have lost its association with hiding. So your Ally (also a new rule) can't see you if his passive perception isn't high enough, and he doesn't make a Wisdom Perception Check to spot you.
6e Fun fact: all NPCs are now monsters.
"Rulings not rules" has clearly been laid to rest. Another 38 rules have been "revised." You don't have to call it a new edition. Revised 5e might be appropriate, but 6e is so much faster to type, and not any less correct than R5e or even 5.5e.
PS 6e Fun fact: all beings are creatures. No mention of what a being is, though.