Hearthpwn

I haven't checked statistics or anything but I do think that a mechanique like Plagues always needs some way to counter it, its a really anti fun mechanique that in alot of matchup pretty much is a free win.

so are most OTK decks.. and thats apparently intended

Unless you play a very slow deck with no gameplan, plagues will not impact you that much.

I hate playing agaisnt plagues - the design irritates me, much like curses did for warlock (especially when they're infinite). 

I hate the randomness.  You have your turn planned out then - bam - frost plague.  I watched a Kibler video and after trading to isolate the primus with 17-odd health he played Dubious Purchase, only for the draw component to go first, draw two unholy plagues and the card to kill them instead.

All that said - are they OP?  Probably not.  There are ways to play around them with some classes and Tony is available to every class.  From a DK perspective I suspect you're always sitting there thinking - why aren't they drawing plagues?  Their whole deck is plagues!!  If Helya gets played then plagues ultimately become a win-con on their own but that only matters in very slow match-ups.  Other than that they tend not to drastically influence the outcome of most games.