All of League is too Win-Loss based. (Includes Letter to Riot)
EVERYTHING is win-loss.
What determines your initial rank? WIn-loss. Why people get into Gold when they suck: People carry them. They win. When they play on their own, they make everyone in their game mad, and that leads to overall chaos in the rank system
What determines IP gains? Win-loss. Has nothing to do with your K/D/A. If you're farming for IP might as well go 0/10 b/c you will still get the same amount no matter what.
What determines Mastery Score (Not letter grade)? Win-loss. You get NO mastery points if you lose (like 200 or so, but it is nothing). Even if you played a better game than Faker has ever played, you still get a max of maybe 250-300 if you lose.
Riot, this next part is aimed right at you. This is a polite note, and I am not trying to rage or vent, as I am not angry about this issue. This could easily be fixed and would improve League a lot. This not only covers the above problem but also a problem with Trolls.
I think your game is too focused on "Win or Loss", maybe because that's all that matters in competitive play such as LCS and Worlds. The general logic of your game is "If you do well, you will win." This is definitely not true. You can go 30/0 as jungle, but if everyone else is feeding, you can't do anything about it unless you're capable of going in 1v5. I think you are pushing this "Dynamic Queue" and "Playing with Friends" way too much; everything now, and any new features you have been adding, have been win-loss centered, implying that we should play with a team to do the best in our games. Some people, however, don't want this. It leads to many issues with a large portion of your player-base. Personally, I don't like playing with my friends b/c they don't take the game seriously, or taunt me mercilessly when I am behind or having a bad game. I can get very, very toxic and I don't like this because it may end up in me flipping out at my friends and losing a real-life friendship; so I usually avoid everything but soloqueue, because if I am in soloqueue I can just mute my teammates if they are giving me trouble. However you have been slowly closing this game off to solo players and I am afraid that one day you may go too far with something, and lose a massive chunk of your profits and players.
Back to the win-loss aspect: It really isn't fair to have everything based on whether your nexus or their nexus is destroyed. A few hours ago I went 11/6 with Vayne: A very good game where I ended up dying a lot at the end, but I overall performed pretty well (This was Normal, not Ranked). However we had issues with a feeder, a Wukong Mid, and a Kayle who went AFK for about 5-10 minutes (which threw off my lane phase hard). We ended up losing even though I had very good kills and some crazy escapes. I expect some sort of compensation for "hey, you did well, even though you lost." However, what do I get? 100 IP (Mostly b/c the game ran 35 minutes) and 200 mastery points. I got a B+ rating, but that was the only thing that wasn't based off of my loss. PLEASE make mastery score somewhat more proportional to your performance. Even 50% of a win would be nice.
Players repeatedly see this in ranked: Losing 22 LP on a loss, but gaining 16-18 on a win. Just on paper, this sounds very unfair. What also is unfair is that LP losses INCREASE with a lower MMR. It definitely seems to be a better option to flip it around; making it harder for players to get promoted but easier to come back from a losing streak. Also make it based on KDA, somehow. Carrying a game and losing to a very lucky Yi backdoor, which you just didn't see, doesn't deserve -22 LP. Losing because of a troll feeder (my next point) should be much less punishing, or maybe even loss-prevented.
My final point is with the trolling problem in League, especially Bronze. By now you should have seen some pretty bad forum posts about some sort of trolling and how Bronze is "ELO Hell". I hope you have seen this before because it is almost impossible to miss. ELO hell does exist, and it is pretty bad. I am in Bronze 2 so I probably don't get the worst of it but I know there are some horrible cases lower on the ladder. Trolls punish everyone. They make the game horrible to play (and you can't leave or you'll get a LeaverBuster), they make you lose 20-25 LP, and they discourage you from playing League, which means you lose precious PROFITS. I have heard rumors about a loss-prevent system for AFK and trolls, and it is desperately needed, true or not.
If you have read this, thank you. If you aren't Riot, comment +1, or even -1 if you disagree, to bump this to Riot.
I will probably bump this periodically; this is very important.