I have had countless roleplayers (All of which were blood elves, ironically), who think that levels has nothing to do with roleplay, and I am here to refute that thought process with pure logic.
Here we go.
Why your level 50 cannot beat my level 90:
Let me start off by saying, I HATE RP dueling. I -hate- it. It always turns into a 'Nuh uh. I hit you' 'No you didn't, I dodged it!' 'Nuh-uh! You can't dodge my...super...mega...hyper...awesome...destroyer beam'. Unless it's in a controlled environment, as in we sort of have an idea of what we want, then I -HATE- RP dueling. If I have to determine a physical victor, I prefer to duel and let whoever wins...Win.
Here's the thing though. My character can probably take on 10-15 level 85 Jinyu in Pandaria. Does this fact matter? No. What -does- matter is that your character cannot. I am even willing to turn into the Obsidium Nightwing, let you mount up, fly you to the area (if I can), and watch you fight this mob.
My character is decently geared for its level, and even if it was in greens, the stats they give would make it so in a duel, your character would get -stomped-. But let's take away the stats. Let's just go full RP.
Your level 50 character cannot beat the mob that drops the item. Let's go another angle. Let's say your character is a trade baron and buys the item. Awesome! They can't equip it.
'So what? That's just a gameplay gimmick' you might be saying, and you know what? You're 70% right. The other 30% is pure logic. Do you remember how warrior have to wear mail till level 40, then they get their plate? Why notjust scale down plate stats to mail level, call it all plate, and let warriors wear that? Because it doesn't make any sense, that's why. Plate is heavy and stiff. It's hard to move in, and even harder to perform actual combat in. Your character has to reach level 40 to 'gain a mastery in it' so they can utilize plate. By this standard, all forms of armor require some sort of knowledge to be able to equip. All leather is not the same. There are different clasps and buckles. The material may be more stiff or lighter. It may be heavier. It's different, therefore your character needs to grow and adapt to be able to properly use it.
Experience? You mean those purple numbers?
Experience, also, matters. In game it is represented by purple numbers whenever you kill a mob or complete a quest. But it's also more than that. Here is a real life example: Let's say you have a family member who has been in the construction business for more than 20 years. If you and that family member were given the task to build a house within 4 months, that family member would -wreck- you. Is it because they were born with this talent? No. It's because they've been at it longer. They have Experience. A level 90 has seen more and done more than a level 50. It's just that simple.
My character has beaten Deathwing. I've gotten the achievement for it and everything. For roleplay sakes, however, since that is clearly far too epic of a task for him to have done alone (or even in a group of 10), I prefer to state that he's beaten a liutenant of Deathwing, likely a member of the Twilight Hammer. It's small, but it has its own merit, and it's something a level 50 could not do.