Here's how I explained it to Konata the other day:
Keep it realistic, as in, only play cards that people could actually afford in real life. So being realistic, no moxes, no power nine, no cards that cost $300-$4000 each. Apart from that, infinite combos, really awesome combos I'm all for--because that's where the fun is. If you can pull of your infinite combo before I run over you with my little creatures, fair game, that's the fun of the game.
Unless of course, you and the person you're playing are in agreement that you're playing power nine cards and the deck is merely just for fun and for an experiment.