Actually, you’re completely wrong here. Rexxar was good by itself after they updated the beast pools (and pretty bad before). It literally made aggro hunter late game viable (it was two decks in one, aggro and Rexxar). You might be thinking that Vicious Scalehide was in there originally or Bloat Bat or that rush poisonous minion, but they weren’t. Rush and lifesteal were the game-changers.
If Frost Lich Jaina were in standard right now, it would be super powerful. Just like Dr. Boom wasn’t really problematic when there were more broken cards available. Broken cards don’t seem that broken when they are surrounded by super broken cards. Make sense?
And decks that perform poorly can still be problematic if they are excessively polarizing or if they are insanely good in the hands of the best players (Quest Rogue, Patron Warrior).
Most Death Knights were gamebreakingly strong. That’s why they had to print broken cards like Genn and Baku so that the next set would have an impact. That’s the problem with printing cards that are too strong. You have to keep printing stronger cards.
Here’s an interesting side note. Bloodreaver Guldan would actually be pretty bad in standard right now.