Sorry for the late reply, but when I said casual, I meant that players playing Dominion don't really feel the same pressure of self-worth that is in Summoner's Rift, where your own decisions/skills could really mean the game. You can't run to a turret and disable its agro by simply capturing it, either.
However, Dominion strays from that. Instead, we rush towards capture-able towers in a map filled with skirmishes. Roles lose their real purposes. Kill them, take points, it's a very simple game mode. In fact, it is literally the moba equivalent of a fps capture-the-hill game mode.
This does not mean it is a "bad" game type. I find it fun to play it sometimes, but honestly, I don't really care about wining or losing in Dominion because I cannot bring myself to take it seriously. It's a game mode to relax and take a break from the precision needed in Classic mode.