First off, allow me to point out:
Create a voluntary review process where someone can say "Why did I lose this match?"...
The question of "why did I lose this match" is arbitrary and pointless. Sure, you could potentially learn what you could do better if you knew how the match was lost, but ultimately, the reason a match turns to a loss can be any number of reasons. Chasing kills instead of objectives, failed suicide-splitpushing, a lost fight at Baron, etc.
If someone really cared why a match turned out to be a loss, they would review the match themselves to try and understand what went wrong.
...and players can submit responses like the board post, and answers with the most votes go to the top of the page.
Players earning X number of upvotes over Y months earn loot.
"I want to get some free loot, so I'm just gonna find some random player looking for advice, drop some random, vaguely accurate comment, and then hop on alt accounts to spam-upvote my comment to get free loot."
D'you see how easy it would be to abuse that idea? If you're going to try to use free content as incentive for players to do something, you really, really need to foolproof the idea so that it can't be abused.