What is to stop a young Tauren brave from wearing the mighty gear of his elders? Get your head out of WoW for a moment – what is to stop him from putting on the physical garb which conveys these great bonuses? What prevents him, exactly, from picking up an epic mace dropped by Deathwing himself and smashing opponents around him? Okay, let’s say the mace isn’t from Deathwing. Why not a level 60 mace? Or a level 35? How does that work, exactly?
Because a level one character has not yet developed the mental of physical strength to bear such items. These examples aren't just higher quality. They are magical. Magic does weird things. We know this. Magic can tear a person apart. It can drive them insane. It can unmake existence itself, on varying scales. Keep that in mind when you ask why a level one cannot wear a level 60 helm.
On one level, it may be a game mechanic, not a "lollevelupnub" mechanic, but a "we're not going to let you equip gear which will make your character instantly die" mechanic. There used to be an item that did that. It was an item for a fedex quest, but, but it could be equipped. It killed you if you were unworthy. You were unworthy. The item can no longer be equipped. On a side note, I think it would be great if players could equip higher level gear, with ever-increasing chances to cause serious harm to themselves.
Going deeper, we could see how it would be physically impossible to equip the item. A 'good' item might not let you equip it to protect you. Or to protect itself, because it's not generally a good idea to give powerful magical artifacts to people who don't have the ability to wield them, let along protect them from evil. Alternatively, a 'evil' item might not let you equip it because you're weak and unworthy. You are beneath it and may only ruin its evil plans.
Sadly, WoW never bothers to explain this to players. No NPCs, no quests, no cutscenes, or books are there to tell us that trying to use items that we cannot can have dire consequences. Oh sure, there are some hints here and there. The red text suggests danger. The reactions of group members when your hunter rolls need on everything are a sort of community warning. Yep, we're not angry at the hunter, we're worried for him, like when an infant finds your rusty nail collection. While you are upset that they might damage your precious collection, you're more worried about them harming themselves. Babies are such noobs.
It's about time Blizzard fixed this oversight. I don't expect a detailed explanation of exactly how magical items can kill us, I'm not a nerd or anything, but just explain a little why the item text is red.