Cards that are so good you can stick them in any deck and play them in any situation are worse for the game, neutral or class specific doesn't matter. They have to keep these under control. Keeper of Uldaman is the newest offender here, it goes into any paladin deck, aggro, control, secret etc. These cards reduce variety so I feel they're bad for the game.
Strong cards that enable a new decks are actually good to me. MC is just too strong. We have Goblin Blastmage or Wyrmrest Agent and a lot of others for example. They are very strong for their cost but enable certain decks, they don't go in any deck and you don't win the game when you put them on board like MC. These cards can bring other cards into the game that otherwise won't see play. like MC or Grim Patron have. Grim Patron is a well designed card with that stat, cost and ability. MC isn't. I like this approach overall, you just have to be mindful of how strong the card is.