Li Shenron
Legend
My first thought was: "Where are the modules"? 
Most of the stuff in the DMG is "modular" at least in the sense that it is "optional" for playing the game. But we were told that the DMG was almost like a toolkit to ADD or CHANGE rules on top of the game, so let's see how it does that when we have the book at hand...
I have been writing about this several times on ENW but nobody noticed
I casually asked Mearls on Twitter months ago if there was any chance for having chase rules in 5e core and he replied "You'll see them!" so I am happy he kept his word 
My hope is that there are really a lot of "sidebars" for those, scattered across all chapters, for modules/options that are really so simple they don't need a lot of space with explanations. But which are those "25 pages of optional rules" you are considering here? I was thinking that e.g. the sections "Wilderness Survival", "Urban survival", "Downtime activities", "Alternative Rewards", "Ability options", "Adventuring options" and "Combat options" belong to that.
All in all, I think the 5e DMG was planned to be non-essential to play the game, so maybe it could have avoided carrying too much detailed information about "how to run the game". That's more like beginner's stuff, and I thought the DMG in 5e was not supposed to be for beginners. I would have thought it was going to be a lot more about "tweaking/customizing" and looking "behind the curtains" to understand more about how the mechanics work and what happens when you customize them.
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But all in all, my only disappointment is that apparently the DMG doesn't have the announced system for running large battles. That's not the same as "mass-combat rules" (i.e. running battles between armies) and also not necessarily the announced Battlesystem. Those would be nice to have, but they are rarely needed IMXP, unless you play a very specific type of game.
Instead, LARGE battles are simply battles when your PCs are surrounded by ~20-30 smaller foes, like a band of goblins or a pack of wolves or a mob of zombies. These are not battles between armies, your PCs are still individual characters, but running each monster individually is a pain. Still, these are very common scenes in fantasy!!
I don't care what kind of solution they propose in 5e (swarm rules, mob rules, battlesystem...) but I really hope this is hidden in the Running the Game - Combat or Dungeon Master's Workshop - Combat Options sections!

Most of the stuff in the DMG is "modular" at least in the sense that it is "optional" for playing the game. But we were told that the DMG was almost like a toolkit to ADD or CHANGE rules on top of the game, so let's see how it does that when we have the book at hand...
2 p. chases (have we ever had that in a core book?)
I have been writing about this several times on ENW but nobody noticed


30 pages on world building
25 pages on the planes
55 pages on making adventures/NPCs
95 pages of magic items
25 pages on running the game
25 pages of optional rules
10 pages of random dungeon design
For a game built on "making the game your own" and "playing how you want", 25 pages seems pretty tiny. Like super tiny. I hope there's more customization content spread out over the rest of the book.
My hope is that there are really a lot of "sidebars" for those, scattered across all chapters, for modules/options that are really so simple they don't need a lot of space with explanations. But which are those "25 pages of optional rules" you are considering here? I was thinking that e.g. the sections "Wilderness Survival", "Urban survival", "Downtime activities", "Alternative Rewards", "Ability options", "Adventuring options" and "Combat options" belong to that.
All in all, I think the 5e DMG was planned to be non-essential to play the game, so maybe it could have avoided carrying too much detailed information about "how to run the game". That's more like beginner's stuff, and I thought the DMG in 5e was not supposed to be for beginners. I would have thought it was going to be a lot more about "tweaking/customizing" and looking "behind the curtains" to understand more about how the mechanics work and what happens when you customize them.
---------------------------------------------
But all in all, my only disappointment is that apparently the DMG doesn't have the announced system for running large battles. That's not the same as "mass-combat rules" (i.e. running battles between armies) and also not necessarily the announced Battlesystem. Those would be nice to have, but they are rarely needed IMXP, unless you play a very specific type of game.
Instead, LARGE battles are simply battles when your PCs are surrounded by ~20-30 smaller foes, like a band of goblins or a pack of wolves or a mob of zombies. These are not battles between armies, your PCs are still individual characters, but running each monster individually is a pain. Still, these are very common scenes in fantasy!!
I don't care what kind of solution they propose in 5e (swarm rules, mob rules, battlesystem...) but I really hope this is hidden in the Running the Game - Combat or Dungeon Master's Workshop - Combat Options sections!