Two more things that I see as problems:
1. The encounter building advice in the DMG is entirely based on answering the question "I want to make an encounter that's (easy/medium/hard/impossible) for my party of level X. What monsters/traps/skill DCs are about right?" Where what I think a lot of people want more advice on is "My party of level X is in situation Y. How hard is it for them to get out of it?" For example, I remember reading one thread somewhere where the OP asked "How do you stat out the town guard?" and the advice was along the lines of "Well, if you want them to be minor nuisances, make them minions, if you want them to be a tough fight, make them a full combat encounter, etc...). But part of the problem is that in that situation, that assumes that the DM has some predefined role that they want the town guard to play, when in reality what some DMs want is something that will allow them to figure out how hard it should be based on game world considerations.
Why is this hard? Just invert the monster guidelines. "I want the guards to be level 5 soldiers; there are 6 of them in a typical patrol, so that's a level+3 encounter for my party of five level 3 characters."
Minions might be different because of the scale of the threat; simple rule: if a monster would be >=4 levels below the party, convert it to a minion at +8 levels instead (which keeps the ~same XP value). Edit: the same should also be true of monsters that would be too many levels above the party as envisioned- convert to Elites and/or solos. Actually making conversions, or finding monsters to re-skin, could be a bit difficult. These kind of conversions being necessary is kind of antithetical to a sandbox game, but given the +1 attack/defenses at each level, the system simply won't work for party level -6 or party level +9 monsters otherwise.
Do you want the DMG to set standards like "town guards in general are [this powerful]"? It seems like the DM is better suited to that, though specific adventures/settings might have descriptions of what each town's guards consist of.
(a) Masterwork armor. It's needed to allow heavy armor users to keep pace with light armor users following stat bumps, but it's caused probably more confusion than any other element of the game.
(b) Non-weapon/implement attacks like Dragon Breath need that tier-scaled bonus (+2/+4/+6) to remain competitive at higher levels.
(c) Things like grabs, bull rushes, and improvised maneuvers become very hard to hit with at higher levels, because they don't get a lot of the boosts (like item bonuses) that regular attacks get.
(f) All of the so called "feat tax" feats - Expertise, etc. (Those have been discussed to death elsewhere so I won't say anything else about them.)
You've probably seen proposals along these lines, but here's a math fix house rule that gets (at least partly at) the above issues:
1) At levels 5, 15, and 25, characters get +1 to hit and to each defense (including AC).
2) The feats Weapon Expertise, Implement Expertise, Focused Expertise (to-hit bonus) and Robust Defenses/Epic Fortitude/Reflexes/Will (Fort/Ref/Will bonuses) are banned.
3) Masterwork Light Armors do not exist.
Masterwork Heavy Armors are as follows:
+1 additional bonus to AC for heavy armor with a +2 magic enhancement bonus
+2 additional bonus to AC for heavy armor with a +3 or +4 magic enhancement bonus
+3 additional bonus to AC for heavy armor with a +5 magic enhancement bonus
+4 additional bonus to AC for heavy armor with a +6 magic enhancement bonus
This is a math fix for well known scaling issues; characters lose 4 to hit (compared to monster defenses), 2 to AC (compared to monster to-hit), and 4-7 to FRW defenses (compared to monster to-hit) over 29 levels. Doing it this way also removes the need for Masterwork Light Armor, which was an ungainly fix in the first place (I see the need for Masterwork Heavy Armors to keep up with ability scaling). Note that AC in the default rules, using AV/PH II MW armor, still falls behind monster to-hit by 3 for many levels (even by 4, if you don’t get +6 armor until level 28), while in my fix it never falls behind more than 2 if you get better enhancement armor at levels ending in 3/8.
If you dislike the concept of Masterwork Heavy Armor, you can make this extra AC bonus an inherent part of the enhancement bonus for magic heavy armors. Same effect, different flavor.