Not Sure Of:
Use of Attributes in lieu of Saving Throw categories: just not sure, want to see how it works.
Advantage/Disadvantage mechanic: seems like it precludes the cinematic “succeeding beyond your wildest dreams against impossible odds”. I'd make the 2 dice thing optional and instead give the DM guidelines for determining a light dis/advantage or heavy dis/advantage with a +2/+4, or -2/-4 modifier as suitable.
Themes: I just don't get them. They seem harmless now, but I foresee WotC getting carried away with canon themes as designers make all kinds of fun themes for their favored classes while leaving other in the dust. Saw this in 4e with all the “cool” stuff going to the fancy classes while the core 4 were almost ignored.
Death and Dying mechanic: I understand that the higher you go in levels the harder you should be to kill, but this seems to be pushing it to an extreme. +10 is way too high. Cut it to +5. Otherwise characters are too prone to being “death proof”. I know they can die, but this mechanic makes it too easy to survive, and part of the thrill of adventure is the risk of death.
Restore the Magic Missile to 1d4+1 PER LEVEL. Make it a 1st level spell. Essentially, restore it to 1e/2e standards.
Spending Hit Dice to heal is a just sneaky way to keep those damnable Healing Surges in the game, isn't it? (Yeah, I don't care for Healing Surges.)
Like So Far:
The return of the electrum piece! Don't ask why this gets me excited, I just like saying “electrum”. Lots of fond, goofy memories of my players referring to the "rectum piece"!
Emphasis on simplicity. I saw many guests at my house get intimidated by the rules and the sheer amount of books.
The so far lack of emphasis on minis. Lots of questions from potential gamers asked whether or not they would need a pantload of money just to buy the “right” minis and tiles.
NO POWERS!!! Dancing in the streets on this one. Let the spellcasters be spellcasters! No need for gobs of special “powers” for everyone just so nobody feels left out. So the fighter can't cast spells. Big whoop. The wizard can't bury a telephone-pole sized falchion in your skull, either. If you want to swing a sword and cast spells, multi-class.
Like the way rituals are handled. Keeps a cleric or wizard from having to memorize spells that will most likely be used in non-combat situations, thus cutting down on the number of spells that might actually help keep them and their comrades alive.
Thank you for finally defining Mage Hand in the manner I believe it was intended. My number one player, my husband, disagrees and doesn't care for it. He believes that if you can find a creative and potentially damaging way to use a cantrip you should be able to go for it.
Invisibility says you can still be spotted by a potential shadow. How does becoming invisible affect even casting a shadow? The light is bent around you, so no shadow should be present.
Why would a heavy crossbow use a STR mod? The mechanical parts of a crossbow only need your STR to effectively **** them. I can see needing a particular STR to use one, but not to add to any attack score.
Sling damage: Is that for a stone or a sling bullet? A sling bullet is specifically designed to deal greater damage than a sling stone.
Sling staff? Common enough technology, contrary to current misconceptions is perfectly fine for use in dungeon-type environments. Don't believe me? Watch LaCrosse. Short, powerful throws, not always requiring a large arc. Unless they're thinking of a traditional sling attached to a traditional staff (which I admit is unsuitable for dungeon use) please consider including the LaCrosse style sling staff in the final equipment list.
Don't go crazy with classes. Before you create a whole new class, ask yourselves if it can be a sub-class for one of the core 4.
Keep the release of books at an affordable pace and price, such as one every two months, ranging from 15$ for soft-cover minor supplements to 30$ for hard-cover settings and heftier supplements. Modules shouldn't be more than 15-20$ depending on size and material added. Get more mileage out of the game that way. Feed good teasers to keep people from paying too much attention to third party products.
To call DC 10 “Trivial” sells that DC short, especially at lower levels. The text further says it's “so easy that it's not worth a check”, and that an adventurer “almost always succeeds”. Save that for a DC5. Let's not discourage DMs from calling for a roll that might sound like nothing but can be nasty if failed. For example, seeing someone's reflection in a mirror behind a bar counter is easy, even trivial, and certainly not worth a Hazard assignment. However, it should not “almost always” succeed. A hero might not notice the guy walking up behind him who he helped put in jail ten years ago but who recently broke out and is looking for revenge. I know, I know, DM fiat, but I'm concerned this definition is being too dismissive of a trivial matter that can turn very exciting... or ugly.
Drawing a weapon is incidental, but what about switching weapons in a fight? Dropping your pole-axe in favor of a short sword as the enemy gets in too close, is that also incidental?
Again, these are just first impressions. I just got the thing last night and was worn out from a hard day at work. Comment as you like, help me catch some things I might have missed!