My biggest complaint about the class now is that they have 3 making mechanics (Mark, Challenge and Sanction), which can be a pain to track.
Challenge still has the unfair downside that it's pretty easy to loose it (and the real problem: loose your shot at being able to use it again on your turn) when the paladin gets stunned/immobilized/dazed et cetera. I was really hoping for a feat that removed the adjacent requirement (more often than once a fight at paragon) or the "Can't mark on your next turn" punishment...
Since the feat ADDS Str, strength based often can deal more damage with challenge (and there's a viable and potent Str/Cha build now too), at the cost of their sanctum kind of sucking (always Cha based).