But that turned into a 3000 word stream of consciousness debacle. So here's some wild-ass guesses.
- 15th level Maximum. 16+ saved for an expansion. No spells above 7th level for anyone, other than in the epic book, which breaks the game in half like 16+ always has.
- Casters that can only prepare one slot per spell level at a time. This gives you "encounter powers", only you can keep picking new spells through the day. It limits NPC complexity and advantage. Cuts back nova-casting. Bla, bla, good.
- Everyone gets +1 to everything per level, and all the monsters do too. Saves are backward like 4e, but durations are simpler (all sustained by the casters with their limited actions). Long-duration buffs become Wizard feats instead.
- Feats that are stunts/spells, with no prerequisites other than class and level, like 4e Fighter powers, but nothing ever stacks with anything. So you either use your Cleave feat, or your Whirlwind feat, or your Trip feat, but never more than one per round. Sorcerer is a feat-caster, Fighter a feat-sworder.
- Everyone gets a 5-level prestige class at 6th level, but they work like Kits (aka Paragon Paths). There's also other 5-level "epic destinies" you get at 11th. So the core classes are a bit boring, and everyone gets their flavour from the Kits.
- Fights that last 3-6 rounds, usually 4. So monster hit points that aren't stupid.
- Five or ten monster classes, like Dragon, Outsider, or Giant. So a storm giant is a 15th level Giant with some spell feats, and they give a couple of examples of each type, with custom feats for the odd ones. Horde monsters are low level and have trouble hitting the PCs, which is a good thing.
- PCs with Con score + d10/level hit points (max 1st, optional 5/level). Fighters that get bonus hit points when being healed, everyone with limited personal healing (like a 3e Monk), but also most classes with some team healing capacity (like a 3e Cleric).