DnD has a long complicated relationship with math and what math means for how the game works. I’ll be honest and say up front that one of the things I like best about 4E is how predictable the math of the game is, and I think that is a design feature of 4E. I have played BECMI, 2E, 3.X, 4E and Next and each of those editions has a different relationship with the underlying math of that game.
One of the things I like about Next is the way the range of expected values for a task is intentionally narrowed, the bounded accuracy idea. After so much investment in 4E though sometimes it leaves me struggling, such as the level 10 dragon that is easily hit by the level 1 fighter. The dragon will win because its hit point bag is bigger than the fighters. This isn’t a bad thing overall however as it explains why dragons and similar creatures need to be worried about the 50 guys with weapons over there, which is the design goal.
However for this idea to work the underlying math needs to work as well. If level 1 creatures have a generic AC of 12 then because every 4 levels the average attack has gone up by at least 1 the generic AC should likewise increase, so at level 8 be 14 and level 20 be 17. These are still well within the bounds of “hittable by the town watch”. Sure the 5 guys on patrol probably cannot hit the AC20 level 20 monster, but a lot of them attacking are going to get some shots in still.
The problem becomes much clearer when you look at saving throws instead of attacks. A level 1 caster should readily have DC14 for their saves, the right feat means making at least 1 save vs a critical spell with disadvantage. This doesn’t seem like a problem until there are level 10 monsters that are more likely to fail that save than make it. Monsters for whom the ever increasing DC of the saves just makes the situation worse; by level 10 the save DC should readily be 17. The absence of any level adjustment on saves is crippling these monsters. 2 level 10 clerics loaded up on Command can easily make an encounter with a single tough foe trivial. Even giving the monster advantage on the save doesn’t help when the base roll it needs is a 16 or higher.
This isn’t just a problem for monsters either, PC saves improve only marginally even for their core abilities across 20 levels. Its the first edition of the game were this is true. The Wis 10 fighter is just as likely to be charmed at level 1 as at level 20 by the first level wizard, but the 20th level wizard is pretty much certain to charm the 20th level fighter, because the wizard’s save DC has climbed easily to 20.
This disparity was fixed in 4E by the balance in the math, while earlier editions had more haphazard advancement they still gave advantages for higher levels and Next needs to address the issue as well. They do not need to make the flat progression of 4E, but something must happen to redress the math issue before the game is finalised, otherwise the imbalance in the math will weaken the foundations of the game as a whole. The math matters and taking the time to get it right will make the game better as a whole.