Starcraft 2's matchmaking system is pretty damn good at finding you equally skilled opponents, which is both a blessing and a curse. Learning one race takes months and months of dedication, during which you will slowly advance through the ranks and face off against players doing the same, but should you decide to switch to a race you know nearly nothing about... well, that's where the problem lies.
While you might be a Diamond Protoss player, your rank rank as a Terran could potentially be somewhere around Gold, purely due to the different mechanics and playstyles the races require. As of right now, your options are either to get completely destroyed over and over again until you fall to your proper rating with the Terran race, or to simply never play them on the official ladder which means your skills will improve at a much slower pace, further propagating the problem. No matter which way you go about it, the end result is quite clearly not ideal.
Thankfully, Blizzard has finally decided to do something about it! The next Starcraft 2 update (3.7) will essentially separate your MMR (matchmaking rating) by race, so you'll never again need to worry about swapping over to your 'bad' races. If you're wondering how all of this works, here's a brief rundown from Blizzard themselves:
"When you first login after Patch 3.7 is applied, there will be a new season for which you will need to play five (5) placement matches before being placed for any given race, even if you placed into the ladder with that race during the previous season. This will ensure that players who played as multiple races prior to this patch can receive accurate rankings for each individual race.
As you complete your placement matches, you’ll receive a ranking for each of the race options. This rank is completely unaffected by any games you play as another race option, including ‘Random’. We’ve opted to give the Random race option it’s very own ranking as well, since people can identify with being “a random player” and we wanted to preserve the meaning and value behind choosing this race option."
It may have taken them around 20 years to finally implement this quite obvious change, but I'm still incredibly happy they have done so. At least now I'll be able to play the Zerg without getting absolutely obliterated within minutes!
Do bear in mind that this will only affect ranked 1v1 matches, the 'serious' competitive mode, while everything else will remain as it is. I'm not sure if they are simply going to skip the other modes or if similar changes are coming in a future patch, but it would certainly be a welcome addition as the same problems apply in team games as well.