It's worth noting that Prince Keleseth wasn't played very much at launch. It took several weeks before Rogue started to abuse it.
The Caverns Below deserves a mention, but If we're talking about a disparity between expectation vs reality, then To My Side! takes the cake. It had recieved 70% "Dust It!" votes upon reveal, then proceeded to be used in a tier 1 deck. Not even Backstreet Leper got that many "Dust it" votes.