For the most part, we see that the single most powerful monster cards in Hearthstone are legendaries e.g. Ysera, Jaraxxus, Ragnaros.
However there are some exceptions such as Finkle Einhorn and other cards such as Nat Pagle which are worse than some class specific cards such as mana tide totem which gives a guaranteed card next turn as opposed to a 50% chance.
Best Answer
Legendaries represent a unique character, other cards represent a spell or a class/title. This is also why you can only have one of each legendary in your deck, but two of each of the rest of the cards.
There's only one Lord Jaraxxus and one Lorewalker Cho, their names are unique. The other cards have general names only describing them and/or their job, Bloodfen Raptor, Armorsmith and Angry Chicken for example.