Aka here's my more in depth look
Initiative: I like the idea that the check you made on surprise round becomes your initiative, a neat touch.
Size: I like the addition of the "number of creatures that can surround you". Its a simple detail that can be nice to have especially for really large creatures.
Mounted Combat: I do think this area could use a little more polish, especially since you opened the door for halflings to ride their allies in battle for automatic +2 AC. I do think the proning rules are confusing, and I still don't know if I'm supposed to get the "5 ft or higher" bonus while mounted. There's also no notes here for intelligent mounts like a paladin mount, do they work exactly the same or do they get their own complete retinue of actions.
Prone: So you can take a reaction to drop prone against a ranged attack, versus interesting. Gives melee a little more benefit over ranged, which I like.
Dragging an Ally: I'm glad to see rules for this, its one of those scenarios that happens at a table decently often but hasn't always had a good ruleset.
Fall Back: This feels fiddly to me. Its a big penalty, I lose my reaction, and I have disadvantage, all just to remove advantage. Its not like I even get farther away, as they get to freely move with me. I can see the idea of trying to add some more mobility into combat, but this doesn't have the right feel to me.
Press the Attack: Having run a lot of barb players, honestly this is the key reason to play a barb currently. Players love this mechanic. The trick is most classes don't have the durability of a barb to take that disadvantage. I don't know, on the one hand I think some players will literally just use this all the time. But it does improve melee over ranged. But it also removes some barb uniqueness. But it does require a bonus action.... I guess I have to see this in play, its definitely one I have reservations about.
Sprint: So my main concern with sprint, is it feels like 95% of the time its just a flat out superior dash. The check is pretty easy, and even if I fail I don't feel the fatigue until after the fight. Though I have noticed that fatigue does look like it can add up in a fight, so maybe I am underestimating that. But yeah at first glance, I think anytime a player wants to move outside of his dash range, they will sprint with 100% no hesitation.
Tumble: My only beef here.... why does this not use Acrobatics? That is like the main reason people have historically taken such a skill, and the idea of using a saving throw "proactively" here really goes against the normal spirit of saving throws. You don't make saves as the results of your actions, but others....so that's a very strange mechanic. I'm also still not personally on board with maneuver DCs, I still like opposed rolls better.
Unseen Targets: I really appreciate the clause that "if both creatures can't see each other, attacks are made at disadvantage". This removes all of the stupid insanity with fogs not actually providing benefit and things like that.
Two Weapon Fighting: So a 5th level ranger could make 4 attacks a round with two shortswords and full ability score on both. That's pretty darn deadly, but bonus actions do see more precious because of manuevers so its probably balanced over all, even if on paper I think TWF has the highest DPS now.
Critical Hits: I like that crits are cooler (straight up double damage!) but there are more ways to deal with them. That's a great compromise which I think will make fights more cinematic. I approve!
Damage Resistances: It looks like you have opened the door back up for some "remove 5 damage" kind of resistances from older editions. I approve, especially for boss type monsters. I respect that half damage is simpler, and when you have monsters taking a lot of damage, its much simpler math to just get all the damage and half it. But sometimes you just want creatures with that nice static damage reduction, to remove the "riff-raff" damage, so I appreciate that its back as an option. I would just hope its not too overused.
Massive Damage: On my second read I realized you got 1 fatigue/1 strife if you pass the save, and suddenly it clicked. This suddenly means things like big falling damage can actually leave a mark, and big attacks are suddenly scary again (in the base game, many little attacks are way scarier because of their ability to generate death save fails). My only concern is that save or dies just leave a bad taste in many people's mouths, there is a reason we have hitpoints. I might prefer it was 2 fatigue + 2 strife....and then half on a successful pass.
I will say that this makes fireball even more deadly. At 5th level, the chance of a 35 damage fireball is a pretty reasonable chance, and the fact that can kill you outright is pretty darn scary.
Death and Dying: I do like the shift towards fatigue and strife as opposed to auto death fails for many effects.
Fatigue: This section is a bit confusing, mainly around what happens if you fall unconscious 3 or 4 times, is it a fatigue every time?
Doom: Doom is a cool concept, and the fact that 5 fatigue = doomed makes that very scary.
RECOVERY FROM FATIGUE: I'm highlighting this one because I think its the most important. So on my read, Level up is shaping to be a MUCH GRITTIER game than vanilla 5e. While death is a little less likely straight up, Fatigue seems to be common, and very hard to remove....you both need a haven, and you only heal 1 level a day.
Fatigue (assuming its the same chart as before) already gives some pretty strong penalties at its 1st and 2nd levels, so I am worried that players are going to be suffering penalties a lot in level up.
Now of course, some people are going to love more grit, but for people looking to use level up as a "similar to normal 5e, but just better stuff", this may be a big shock to them.
I would highly suggest some alternate recovery versions here (similar to how core 5e offers healing levels depending on comfort). For example, maybe I can remove 1 fatigue by 2 spending 2 hitdice during a short rest, or 3 if not in a haven. Again pretty costly, but it gives me some options. Or make an allowance that on a long rest outside a haven is 1 fatigue, a long rest in a haven is all fatigue. That is more in line with standard 5e experience (which is very forgiving on recovery thats just the style)....it provides a nice lever for people that want grittier, but leaves it less gritty "at base".
My gut just says that right now many groups (including my own) are going to get "fatigued of fatigue".
If nothing else, please make sure there are some key questions about the grittiness level in your surveys....this is very very very important for you to get good feedback on.
World Actions: I LOVE THIS!!! This is something that 5e really needed in terms of more crunch, so this is great. Just a few notes on some of them.
- Forested (Cacophony): A very weak effect, I don't think its even worth tracking as a world action.
- Furnished (clutter): You all really just don't like acrobatics do you? This seems perfect for an acrobatics check, not a saving throw.
Controlling Multiple Summons and Creatures: This needs some rewrites, I've reread it 4 times and I still think I exactly know how this work. The concept makes plenty of sense, it just needs some editing.
Rolling Movement: I get what you all are going for but this section just isn't doing it. This either needs a lot of beef up to really do things like chases justice, or just remove it. Right now I feel like the rules are too weak to justify the page space.
Countdowns: Already used them in a game, already a believer.