This is my biggest argument against it. Because it was TBC. Not Vanilla.Honestly I don't do balancing or design, I simply have faith knowing it's possible since it effectively worked in TBC.
Off the top of my head, problems to your suggestions: itemization was not equal. Spec modifiers would have to change depending on what gear you had. Granted, warriors had a similar problem about being rage starved from having better gear. For example, their Lawbringer (T1) vs Redemption (T3) sets. T1 has spirit at the cost of other itemization points, and T3 has only stam and int. They'd have to re-itemize, at a minimum, all the sets (sorry, holy paladins), or redesign how the classes interact with their stats (goodbye, vanilla feel).
Gaining mana from blocking seems viable, which is what they did with Blessing of Sanctuary in WotLK. I wouldn't mind tanking from WotLK again, but it certainly isn't Vanilla.
Folding mana regen to be deep in ret/prot trees doesn't help. If it gives a limitless supply of mana, they'll just dump a few extra points into the respective tree to pick it up. This will also break pvp, as mana burning pools was a big part of it. A healer that has unlimited mana in pvp? OPOP.
Adding more abilities is now changing the gameplay and the feel of it. Also, in vanilla, there is no such thing as "spec unique." Those changes came later, killing the vanilla feel.
Design aside, spirit = hit also would need to rework the fundamental ways that stats affect players.
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I don't know why I'm coming back to you. You are arguing these points with the knowledge of today. I'm going to reiterate, if you think all of these design choices that make warriors the only viable choice was because of oversight and not intentional, you are very naive. Crappy itemization was an oversight. Pigeon-holing EVERY healing spec to heal was intentional.
Sure, find me a game developer stuck in 2004 and I'll ask them. But I'll humor that statement. They made a lot of bad design choices, I agree. They gave talent trees for flavor. They pigeon-holed the classes to pretty much a single spec. Specs were intentionally below the competitive curve (I won't acknowledge that they were designed to be competitive, however) is what they called the hybrid tax. After two expansions, they've accepted it was a bad design choice and dumped it.
I hope I have evidence for my claims, too. But I've already explained why that is a chore. What's the point of giving paladins a tank spec if they don't expect people to play it? Do you know how many people raided in vanilla? 3%? The game wasn't designed around raiding. That's where the viability of specs comes into play. Here's some quotes on the hybrid tax. They're made in WotLK, but refer back to BC. It was worse in vanilla. http://www.warcrafthuntersunion.com/...rawler-quotes/
Can I ask when you started to play WoW?