My brief overview and response of the original post:
Loot Economy
The interesting part about this opening bit is that it has absolutely nothing to do with loot itself. It's about balancing encounters against strong buffs and the "potency of offensive magic." While most loot involved offers a few buffs to key aspects (abilities, skills, pure combat), there are going to be few Wizards who run around with greatswords, full plate, and rings of immunity. Often, the loot taken (assuming it's spread out and created in a class oriented way) will be more akin to further enhancing a role or a class itself. I'll come back to this in 'Loot Progression'.
Damage Resistance/Soak/Immunity
"I am of the opinion that players should not be able to easily acquire damage mitigation on items." That's a fine opinion to have, but I'd like to know why. Much of what is being offered here as a solution basically dives into needing more loot of more specific tasks that you'll just hoard in a magic bag and pull out when it needs to be put on. Which, really, just adds an extra step or two to having an extra AC or two against one particular enemy. While this might affect casters and other low-strength characters, the people who will be taking the brunt of the damage will often have a high enough strength to make it negligible. That said? I am all for including a variety of options, but that doesn't mean the basic versions should be removed either. +1 Full Plate is easily accessible, and +2 Full Plate should be fairly easy to grab as well to keep up with NWN's basic design. But! There are levels between their functionality and availability where quest and DM loot that's slightly better with a minor tradeoff would be a perfect stepping stone. A +1 Full Plate with an extra +1 against a particular element, enemy, or with a slight buff to go with it but a minor penalty to tag along. It offers variety without breaking the functional elements of the base game.
"It is perfectly reasonable for your character to have some manner of weakness." That's easily handled in the system already. There are bonus caps in place. Items that offer similar bonuses do not often stack. And there's a limit to how many items you can have equipped at any one time. Gloves of Ogre Strength can't be coupled with Gloves of Spellcasting, so you're already having to prune and find your weaknesses. What gear is meant to do is further enhance the benefits granted by your class and build. Staying on your toes and watching out for something you didn't plan for is inevitable. That's why there are potions and wands, to spackle up on the things while offering the solutions as a very temporary option.
Loot Progression
This should be tied to Spell Circles, if nothing else. While keeping things low magic is a fine option, what a wizard or sorcerer can do at level 10 is a massive amount of utility and supernatural problem solving. What a fighter, a barbarian, or a rogue can do at level 10 is combat. Sneak, maybe, and occasionally a skill check. But they cannot attack fifteen enemies that cluster together. They can't cast knock and avoid the trap without trying to unlock it first. They can't summon an animal to tank for them, nor can they heal themselves. So, combat is what they're there for. Removing the level appropriate gear is making them weaker while letting casters continue to have their overwhelming bonuses unchecked. If spells are not also reworked in consideration of loot, removing mid and high level enhancement bonuses just mean everyone should roll up a cleric if they want to do melee damage, wizard or sorc for ranged damage, and bard for party buffing. Fighter might have feats and a barbarian might be able to rage his way into a fight, but without the utility or the capability to end a fight early, they become functionally worthless in comparison.
Much of what is available in 3e, gearwise, doesn't require a specific class or any particular ability to use it beyond UMD for scrolls and wands and so-on. It's an offering to allow a player to build a certain way, to make up for something they are missing, or to really fortify something that they are already good at. Sorcerers will not be using the same magic item choices as a Cleric, who will differ from a Fighter, who will differ from a Monk. Offering more, variation, interesting bits of fluff will always be encouraged, but flat out removing them (especially while playing on NWN) breaks how the game is meant to be played as well as reducing interesting mid- and high-level encounters with their often-times more involved interactions.
I'll gloss over the PvP bit as, again, a wizard will have spell slots and armors and all sorts of tools on his end. A fighter will have to get close to put his +5 weapon to work while the mage pops off a few touch-based spells and they'll be pretty evenly balanced by the time they meet head to head.
Immunties
"I am of the opinion that most (not necessarily all) immunities should not exist on items at all, ever, under any circumstance." Once again, a fine opinion to have. But why? If it was a matter of gear giving an immunity to all damages, to all spells, etc. I could understand your fear of people being just immune to everything and laughing to the bank as they stroll casually through a dungeon and just loot the chests. There are no such items though. Damage resistance items are based upon the creatures that you will face, many of them having damage resistance of their own and the capabilities to bypass it. An immunity can and should remove threats, but it will never be able to remove all threats unless, for some reason, it's possible to have a ring of cold immunity, a ring of fire immunity, a necklace of slashing immunity, gloves of bludgeoning immunity, shield of piercing immunity, armor of crit immunity, helmet of spell immunity, and a sword of magic immunity. But those do not exist nor should they. Generally, immunity items are tied to accessories which mean you can have possibly two or three at a time if you are extremely lucky.
I do see an idea on combating this issue, this non-existent problem, this fear: level-based gold-worth limits. I've seen it on a few servers and didn't understand just what it was about until I've seen your post here. Limiting a gold ceiling amount based on the player's level forces a low level player to be incapable of using +5 Full Plate of Fire Removal or some such silliness. At high levels, that +5 Full Plate of Fire Removal will be fantastic, except there are a lot more elemental threats than Fire and they'll usually be happening multiple times in an encounter with level appropriate enemies with an AB offering a very similar chance of hitting the player as a level 1 fighting his level-appropriate enemy would. That's why CR exists, but that's a whole different discussion.
The Most Important Thing About a Healthy Loot Economy
"To me, a healthy loot economy is one that creates as little of a power gap as possible." When comparing this as a level-range gap, there will always be an issue based purely upon ability score increases, feats, spell slots, attack bonuses, class benefits, etc. Trying to build an event or a quest that accommodates all levels involved will always turn out poorly. The low levels will either be bored, killed, or catered to. The high levels will either be bored or catered to. Events need to have level-limits to encourage appropriate design for them. Inclusivity is nice, but when it leads to snooze-fests or party wipes, it's bad times for everyone.
The bottom line is that level appropriate loot brings classes to a more even playing field at the same levels. A caster will always have more utility and their inherent weakness is melee combat, getting interrupted during spell casting, etc. A non-caster's inherent weakness is not having self-buffs, not having the utility. Their strengths should be encouraged. Reducing the quality/level of loot serves to further cause non-casters to rely on casters and suffer a gold-drain like you wouldn't believe just chugging potions. Speaking of, unless potions are removed, there will always be a way to replicate just what you're concerned of for a few thousand gp.
Personally, as I'd like to see more variety in enemies, in quests, in strong, complex foes. As such, the gear should reflect complexity. I've yet to feel underpowered, but I have left feeling underwhelmed before.