@kerfuffle A big factor (at least for the DPS placements) are, in my experience, the number of deaths and the number of eliminations. But I hope that they value the number of, for example, blocked damage more than the number of deaths when it comes to the tank role, since you're basically dead when a Mei or Reaper gets near you (and that happens a lot because snipers are currently as good as useless)
Also, I don't think a "bad" win leads to a lower placement. It probably influences the placement positively but, of course, not as much as a "good" win