I just saw this quote from another thread
Just throwing this out there, but you only see everyone's elo on the post-game screen at the moment, which has already been modified for the game you just played. Your example has an average of 1474 vs. 1520. That ~46 rating gap makes sense considering one team just won and gained probably 20-30 elo depending on number of games played individually, and the other lost losing about the same amount of elo. That was a pretty even match according to pre-game elo.
Regarding the OP, this is a subject of much discussion in the office. The mid range of ranked elo demonstrates some similar behaviors to what we see in the low-mid range of normal games. This is a complicated issue and a lot of effort is being spent to get the matchmaking experience to feel right for everyone.
The leavers ruin the validity of the Matchmaking score because it often gives one team of players a positive ELO gain when they should have lost, or a negative ELO reduction when they should have won. On an occasional basis, this wouldn't affect the system that much, but since it happens all the time, it's going to heavily alter the score and make mid-tier matchmaking a disaster (which we all agree it is).
The reason that mid-tier Matchmaking is most affected is simple: Most players fall into mid-tier, and therefore the biggest percentage of leavers will be plaguing that category. This is why MM so often results in landslides and players going on massive win or lose streaks, because the scores that each player deserves are being heavily altered by an outside influence that Riot is not doing enough to control.
You guys need to do what Heroes of Newerth did from the start, a percentage based leaver system. If a player has left over a certain percentage of their total games (say 10%), they are no longer allowed to play in Normal or Ranked games until they drop below the threshold again. This amazing system has practically eradicated leavers from all HoN games, and I can see no logical reason why you haven't implemented it as well.
If you really want to fix matchmaking, you need to attack the problem at the source.
edit: This thread is a continuation of a thread I made before located here.