There is nothing more unsatisfying than getting a rare drop off an enemy, only to see "Ha-ha, you need to gain a few more levels to even use this!"
I would have to admit, that would leave a bad taste in my mouth as well, and I can't lie when I say that lootsharing would probably have solve all of my problems. Well that is aside from cards, which I believe should be given to everyone since that crud takes forever to get duplicates of for a friend. Then again I don't think cards should be tradable anyway so that's beside the point.
The reason I feel like it's not a horrible idea is when you play games like Starbound or Terraria when a friend can just toss you the best items craftable and you're just like "well I beat the game," and then you can either do all the hard work of gaining an item that you ALREADY HAVE which is, in my opinion, a waste of time. The obvious other option is to do all the work for that item so you can actually play the game and have fun, which is true you shouldn't stop yourself from playing a game if someone gave you everything.
I also had a proposition to fix this problem, though.
Instead of level, add tiers.
Tiers would only appear in story mode.
For example: When you start the game you're tier 0.
The slime hat and the rabbits foot are tier 0 items, and can be traded to anyone at any time.
However, only when you defeat giga slime (which is early) is when you become tier 1.
Now everything from giga slime boss drops to the monster drops from the sky palace are tier 1
When you defeat Phazeman you become tier two, which his drops (and Gund4m) are also tier 2.
And every new enemy until you fight the next boss would be tier 2 drops, but the boss's drops would be tier 3.
Which, upon killing that boss, you would become tier 3.
This way, as a player, you are never going to be conflicted with 'you can't equip this item' if you obtain the item yourself.
However, if you are given an item, it has to be an item within your relevant area within the game.
If anything, I think items that aren't within your tier should even be tradable to you in the first place.
Not to also mention, you can even make the tier names more funny and creative.
Instead of "Collector Rank one" You can call it "Slime tier" and "Robot Tier" and the liking.