As with all competitive games, Overwatch is no stranger to people trying to mess with the rating system in order to gain even the smallest of bonuses. In most circumstances this isn't a big deal as the rating system is fully capable of figuring out where someone belongs, even if it takes it a little bit longer. However, when you have multiple people trying to play high-impact DPS heroes regardless of team composition, all in an effort to game the system and earn the most bonus rating due to performance, things start becoming a bit problematic.
With that in mind, I am delighted to say that the developers will soon be removing performance based rating adjustments for Diamond tier and above! In other words, nothing you do within the match itself (outside of winning or losing anyway) will impact the amount of points you win or lose at the very end. This should hopefully result in more people doing whatever it takes to win, even if they might not fully comfortable with the hero they're swapping to in order to better fit the team's strategy.
As for why exactly this is being done only for Diamond and above, nobody outside of Blizzard knows for sure, but my guess is in order to combat smurfs. After all, the system needs some way to figure out which player is over-performing/under-performing so that it can quickly send them to their appropriate rank. What sort of impact this change will have on the general ladder, that's something we'll find out soon enough as this update is set to arrive alongside Season 8 in January.
The second change worth mentioning is that the developers will also be tightening the difference between the rating of the highest and lowest player in a match. This is most likely going to increase queue times for higher ranked players, but to compensate it will also increase the quality of games throughout the entire competitive ladder. Unlike the changes above, however, this feature will slowly be going into effect throughout December as Blizzard tries to find a nice balance between short queue times and quality matches.
That's pretty much everything you need to know, but if you're looking for more details you should head over to the Overwatch forums.