Why am I 1300 when my record is 50/50? 70/30?
Let’s turn the question around first. Why is my record 50/50? If your record is 50/50, that means your match-making rating (MMR) is almost exactly correct for you. It means we are finding teams that as near as possible match the ability of your team. It means that you are being matched as fairly as possible. If we matched 1300 teams at 1500 instead, they would only win 30% of their matches. We would rather those teams win half their matches.

But I’m 70/30, what about that?
If you are 70/30 and we think you are 1300, your MMR will climb to roughly 1533. You will then be matched with 1500+ teams, and your record should decrease to 50/50.

Yeah, so why is my rating still going down then?
The team rating will drop until it reaches the average match-making rating of the teams you beat 50% of the time. For example, if we are matching you against 1200-rated teams, and you are winning half the time, it will drop to 1200. If you are winning 70% of the time against 1200-rated teams, then your MMR and TR should move up to 1433.

But I’m not matching 1200-rated teams, I looked these teams up and most are near 1500.
It is likely that those teams have an MMR of 1200, even though their TR is 1500. This means that even though they look like 1500 or 1400-rated teams, they have been playing more like 1200-rated teams this season.

No matter what I do, my rating dives to 1300
If your team rating keeps diving to 1300 while you win 50% of your matches, then you are winning matches against 1300 players.
Being 1300 means:
-- You go 50/50 vs 1300 teams
-- You are better than 25% of all teams
It means that the teams we are matching you against have average MMRs well under 1500.

How will I get to 1650?
-- The definition of a 1650 team in both the new and old system is “Better than 68% of all teams on this BG in this bracket”
-- To be 1650, your team needs to be better than 68% of all teams
-- This means you will beat a team with an MMR of 1500 63 out of 100 matches, and go 50/50 vs. a team with an MMR of 1650

Wait, why 63%? Shouldn’t it be 68% then?
-- Smart question! The reason it’s not 68% is because a 1650-rated team does not always perform exactly at 1650. We take this into account.

But I’m stuck at 1300, I never get to play those higher-rated teams:
-- A 1650 player will win about 4 of 5 matches against a 1300 player
-- If you are at 1300 but belong at 1650, you should be winning 4 of every 5 matches
-- If you do that, your MMR will go up steadily, and you will face harder and harder opponents
-- Once you move your MMR to 1650, your TR will follow

How long will this take?
-- This depends on how much evidence we have that you *were* a 1300 team
-- If your team *Really* is a 1650 team, average case is 40 games
-- But it can go higher and lower. Great luck = 16, Terrible luck = 150

I guess that’s correct theoretically, but it’s very discouraging:
We do believe that we can improve the experience for teams that start at 1500 and go down from there. However, the changes we prefer merit waiting for the next arena season.

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Q u o t e:
While I understand all of what you typed, it also means I'm done with arenas.

Because apparently I made the mistake of playing arena season 1-5 with my friends and most of the time as a protection spec paladin. It was fun, and it didn't matter. I didn't even spend on much, I had a set of S3 healing gear and ret gear. I used them in BGs. I pvp'd with a shadow priest as a protection paladin. We lost. A lot.

Now what I'm finding out is all that time I spent just having fun (and rarely if ever rebuying my teams, I was happy to spend time at 1300 rating before because I could win some) means that I basically have zero shot of ever getting above 1600. Because not only would I have to find a better class to pair with, but even if I rebuy my team I'm going to plummet right back down to probably 1200 rating and then win something like 8 of 10 games consistently to see much of a rating gain.

Great.

At least you could have let me know that I was hurting myself by not trying to be competitive before. Now I might as well level a new character to PVP with. Because for playing S1-4 as Prot Pally/S Priest and going 5/5 at 1300 rating, I'm screwed and will never get enough rating to spend arena points.

Yay.

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You are by no means pigeonholed in the 1300's, it simply means that to rise to 1600 you'll have to be able to beat 1600 teams half the time. As stated above, if you're still 1300 it's because you're still losing to 1300 teams half the time, which kind of means you really should be at 1300, not because a rating system is keeping you there out of spite.