Delver is a weird deck to be so good in standard. With the sole exception of Ponder, everything that really makes Delver decks good is in Innistrad block, but Delver decks see very little play there. The whole archetype is made viable by the existence of one incredibly broken interaction between two 1-mana commons. And not just viable, but completely dominant.It'll be interesting to see if there's a way to make this work post-rotation with any available library manipulation in Ravnica, or if Delver really is the one-trick pony it seems to be.
Both of them could pay the "iron price" for the opponent's permanents, stealing wins as they stole permanents with Zealous Conscripts.
Lack of instant speed, cheap bounce is also a problem. As is lack of good, cheap, efficient counters.