I'm not even sure it's the giant that is the problem; without Defender or Sunfury, that 8/8 body isn't that big a deal when you're already at 12 health. The combo that makes it a taunt is the truly broken aspect of the Molten Giant.
That's a reasonable argument, but a 0 mana 8/8 is great whenever you can either live to next turn or use it that turn somehow. So even without Sunfury/Argus, there'd be combos like Molten-Shadowflame. Plus, if your giant costs little mana, you have enough mana to dump out some other taunt too (Sludge Belcher, Sunwalker, etc.).
Also, Sunfury and to a lesser extent Argus are not particularly annoying to play against in most contexts, despite being pretty strong cards. Molten Giant is annoying to play against whenever it's viable. (I want to emphasize that I'm not arguing that Molten Giant is overpowered, because I don't think it is. It's just one of the least fun cards to play against that's actually constructed viable.)