I've posted here about a couple match ids already, but I want to talk generally about the matchmaking system as well.
In my games trying out Warden (21 so far), I've gotten him every single time I queued for him. This is because I have only him at purple priority and I guess he's just not that popular of a hero right now. This tells me that the system puts priority level above all other considerations when creating matches and will only break from your priority choices when too many other players have the same high priority hero(es).
I think this is a design mistake for two reasons:
1. The system should be balancing your preferences against its abilities to make good matches. If it has a choice between a rotten match with your top preference and a good match with your lower preferences, I would actually want the good match over my top hero preference. Obviously it should still adhere to preferences in some way, so perhaps it could have some minimum ratio i.e. you'll get at least 50% of games with your preferred heroes or something. If the system was even just a little more flexible about how it uses hero priority it could probably create way more decent matches using per-hero MMR.
2. If people one-trick all the time, the system will have very little information about how well the one-trickers can play their other heroes. Giving players less-preferred heroes just a little more often would help the system calibrate.