1) Rating decay ( after 3 weeks of not playing, your char drops 15 points per reset or something )
2) Only 1 character of the same class eligible for end of season rewards. A dude has a, let's say, DK on 3k rating, DK on 2.9k and a DK on 2.7k, his highest rated DK gets the seasonal award, his other two DKs don't get anything.
End of story really. In fact, gonna tweet this to Holinka.
2nd - no (players must always have an incentive)