I have the inkling you do not know what you are talking about.
It would ease me if you were a little more specific, with grounded examples.
As general idea, I understand one can go too far: one will be so weak he will be restricted to his best move only. But you do not give me anything grounded that could tell me common gear is going too far, or that there are a lot of hard fights that have no QuestLog update near them.
It could be the case, or it couldn't: numbers must be crunched. You are not crunching number, you are just telling you are afraid thoses rules might be "killing" diversity without backing it up.
It was made very clear to me that going too far (on the strong side) was very limiting. I did not need to crowd control as much, and sky high Intelligence allowed me to spam my strongest spell every turn (cooldown reduction has a very poor progression formula).
How far can one go to the other side without making the game obnoxious? I do not know, and do you?
Gear, talents and extra attribute points are not really mechanics (some talent such as elemental affinity might be an exception, but my rule doesn't exclude it), they are permanent buffs pure and simple.
You can google "the illusion of choice in gaming" if the subject interests you.
If you feel too afraid, you are free to adjust the rule from "common (white)" up to "magical (blue)" or "rare (green)". The same way the rule with skill tree can be adjusted from "max 3/5" down to "max 2/5" or up to "max 4/5".
Same can be said about saving. Yes it is more challenging when you have to chain multiple fights without saving, and yes it can be more obnoxious aswell. You are presenting them as opposites when they aren't. It is a matter of equilibrium, pick your flavor. One could even decide to never save and finish the game in one go (talk about obnoxious!).
I understand your concern about redoing fights though, but it would help me if you had some examples in mind. I thought about the QuestLog because there are some puzzles in this game that do not require fighting (so a rule about saving after each fight is not good enough, as it will not prevent obnoxious redoing of puzzles). And, again, you can adapt yourself to this rule by keeping easy QuestLog updates under your sleeve (like Blood Stones, or reading diaries).
The economy inflation is not a big problem in the game (even though you end up with a lot of money), and even less so if you decide to restrict quality of gear you wear. From memory, the only things I was buying were skill books (but I could have done without, and use crafting more), the very rare good legendaries that were sold, and some peculiar items in the early game (rabbit paws for the lucky find gear, crafting + blacksmith combo belt and bracers for mere convenience), and of course the treasure maps.
