Without guidance on how not to break it, this is the worst sales pitch I've heard in quite a while. Like okay, flexibility is good but if things are getting thrown out of what just with feats, multiclassing, and flaming swords, I'd expect some guidance on getting things back in whack. Especially considering that no/minimal loot is a radical departure from D&D norms.
Although I'm not clear on the magical items issue, is it that 5e was tested with zero magical items, or that 5e doesn't say how many or which items it was tested under?
What is the guidance you (general 'You', not you specifically) need? Experience. If (general) you don't have the game experience necessary to effectively balance your game when you start allowing all the optional material, then you probably shouldn't be using the optional material yet.
When you buy a new boardgame and the rules include a whole bunch of Advanced rules over the base game... do you include all those Advanced rules on your first playthrough? No. You play the base game a bunch of times until the rules of the game becomes engrained in your head, and then you look through the Advanced rules and decide "Hey, that rule could make the game more interesting, let's add that one next time." And you start adding them all to the game over time once you are comfortable doing it.
And its the same thing with the Optional rules in D&D. You learn to play and run the base game first. Once you are comfortable with that, and you've gotten a pretty good handle on keeping the game fun and at a equilibrium so it doesn't spiral out of control... then you can starting introducing more complex concepts to your game. And by that point you should have the experience necessary to roll with the changes and keep things grounded for yourself. Without requiring WotC to write up all kinds of instructions to make sure you're doing it correctly.
So there's no point in WotC trying to "teach" the way to use all those Optional rules in the book for people who aren't comfortable using them to begin with. One, because there are thousands of different styles, methods, and foci of dungeonmastering for which any attempted "teachable moment" that they might write in the book is completely useless to thousands of DMs because it doesn't match up to what they need to learn. And two... the best way to learn how to be a great DM is by
doing and not just
reading. They want you to try and fail and then try again, rather than think you can just read a couple teaching moments and then magically be able to run every complicated game scenario you'll ever run into. It doesn't happen that way. And WotC realizes this, which is why they do little to even try. It's like putting those "How To Roleplay" sections in the front of RPG books-- for the most part they are wastes of space. Most players don't need them, and for those that do they don't actually help. Because you don't learn to play just by reading it, you actually need to sit down and play.
Heck, that's really why the Challenge Rating system isn't that "good". Because they know that no system of magically trying to teach DMs how to balance encounters over single fights, multiple fights between short rests, and multiple fights between long rests, will actually work and be effective for everybody because every DM runs their fights differently and focuses on different things. Instead, they give you the most bare-bones system and then tell you to run the game and figure out what works best for you yourself.
But apparently too many DMs don't want to take those babysteps, or just don't want to put in the work to actually learn. They want a magical plug-and-play system handed to them that will allow them to run the game exactly how they want to with every bell and whistle attached, and never a mistake to be found. And they get pissy that WotC can't or won't give it to them. Whelp... dem's da breaks. You ain't getting what you want, because WotC knows its not possible, *and* its not conducive to actually learning the ins-and-outs of the game.
