I played a lot of Dominion – until ARAM-mode was released.

IMO, the problem with Dominion was that it became like Twisted Treeline: i.e., there was a way to win. Initially, it was the fastest way to get first win of the day, and the best way to try out new champs. But, as time went on, it became clear that certain comps were good for winning. And that discouraged the kind of carefree experimentation that made the game mode enjoyable to begin with.

I understand that there is a lot of nostalgia driving the upvotes in this thread. But the reality is that, if players are gunning for the first-win bonus, they will try to win ASAP. They will flame. They will spawn camp. And, for those who played Dominion for all the popular reasons, it started to suck.

Ultimately, that's why ARAM supplanted Dominion. If Riot gives players an incentive to win and sufficient control over the outcome, they will ABSOLUTELY become toxic, in a purely statistical sense. If the outcome is sufficiently random, the players may become occasionally toxic, but not in a way that has any statistical relationship with the outcome of the game.