Is it just wins/losses, or does it take into account match stats like CS, K/D/S etc? If I play my role exceptionally well on a bad team, (i.e. either killing or assisting in every single enemy death and dying only once) am I rewarded or punished for it?

Win/Loss is the major contributor. Then the difference in elos of the players on each team.

Currently the only in-game activity that matters is winning or losing. The suggestion about having things like kda or other metrics comes up periodically, but it's simply too hard to define - the correlation between gold accumulation/killing opponents and winning games is actually pretty weak.

For example, there's actually a statistical correlation between having a high kda and being lower Elo. That sounds counter-intuitive, but the explanation people have come up with is that the better players are, the less frequently they die, leading to less opportunities to snowball and capitalize on mistakes. You can actually see this if you watch lower level games - extremely high kill counts are much more common among newer players, even if games tend to be much longer on average.