Even though IGG loves to make users empty their wallets for cards and heroes, at least they do a decent job with balancing said creatures' and heroes' abilities. Except for a few things,,,
The term "Overpowered", is generally considered to be anything that doesn't have a hard counter, or quite simply just "too good". What I mean by "hard counter" is any form of action that directly counters another, with no question about it. For example, Bullseye is a hard counter to Frost Armor, since Frost Armor has no chance against Bullseye. An example of a soft counter is immunity (unless it's immunity 10, in which case it is a hard counter). Immunity is a soft counter against magical skills, but it isn't a hard counter because there is still a chance that the magic can get through.
A perfect example of two creatures that are overpowered in DH are the infamous Kumiho and Kitsune sisters. Commonly referred to as "those bitches who won't die", they both have some insanely uncounterable synergy. First off, Kumiho has Rebirth 10, which wouldn't be too big of a deal if you could just dispose of her; however, disposal doesn't work, since she has Demonization. In fact, if you even try to dispose of her, the creature that you tried to dispose her with will get instakilled. She also has Gemini, which revives Kitsune if she is in the graveyard. And to make Gemini even worse, it works even if she is sealed or frozen. Ktsune is overpowered as well, basically having an ability that is equivalent to Sweeping Blow on steroids. Danse Macabre hits 5 of your creatures, and inflicts Bleed, which goes through immunity, and stacks. To make her even worse, she also has Unbound, and Gemini, to top it all off.
Another thing that I think is a bit overpowered is Venom Edge and Frost Blade, simply because it can't be blocked by Immunity. The only counter to those abilities is Unbound, which is an ability that isn't all too common among the creatures in DH.