Duel Paladin, in my opinion, was already an underrated deck. With so much Pure and Libroom Pally out there, your opponents don't often see it coming.
Barricade turned out to be the perfect new card for this deck, giving you a taunt (or two) to stall out the game a little longer until you can bring the big guns on board.
Testing Runaway Blackwing but, so far, it feels pretty good. If pulled by Commencement it's especially nasty to deal with. It's not sticky like the other BIG minions, but it makes up for it by essentially being a Ragnaros that doesn't go face. Slap a Lightforged Blessing onto it for a big heal.
Archmage Vargoth is less clear cut. Pulled by Commencement he's amazing because he gets a shield AND casts another Commencement. Pulled by Duel! he can die but if he doesn't (he does have 6 health after all) that's another Duel!. So I've changed my thinking on Vargoth as I've climbed up the ladder from keeping it in hand to have more control over its use, to believing the rewards outweigh the risk of putting it back into the deck.
If you're missing one of the big neutral minions you can make substitutions. What you're looking for is big stats and stickiness. Tirion Fordring is a viable substitute with its shied, taunt and weapon as is Evasive Drakonid, which gives you a taunt that can't be targeted by hero powers or spells.
So far I've managed to climb from Silver 10 to Diamond 5 with this deck. We'll see how much farther I can take it.
UPDATE: Tested Snack Run last night in place of Flash of Light and climbed from Diamond 10 to Diamond 5 so I'm going to call that a success. As others have pointed out, not only does it give you heals but also the chance at additional Duels and Commencements as well as other useful spells like Subdue. By and large I prefer cards I can always count on versus cards that are random, but Snack Run pretty much always gives you something useful. The deck has been updated accordingly.