From a purely playability standpoint, the glass ceiling that comes into play when you hit Diamond III is a big demotivator. There's nothing fun about constantly coming up against Maxed Out decks and opponents sitting 10K+ ranking points when you have 3K at best and minimal chance of improving on that.

Once you hit Diamond III, unless you're maxed out you may as well do the daily quest each day and then switch off til the quest resets.