Fill your deck up with discard effects like Demand Answers, a few super-cheap artifacts like Tormod's Crypt, and then a ton of huge, game-winning effects: Portal to Phyrexia, Threefold Thunderhulk, Platinum Angel, Wurmcoil Engine, the works
Not only is making free copies of creatures a terrifying prospect, but the fact that Sin can provide multiple free lands is even scarier, as it provides the opponent with an amazing mana engine
The other three effects are a mix of effects that couldn't be done in just one of the colors
Its the same as if you had a two drop 3/2 flyer that could only be cast if you discarded a card
I will be discussing them as part of the 99, if you want to build dedicated commander decks around them, you can also do that, but then their evaluations can shift dramatically both from them being built around completely and consistently available
Even if you end up having your Heartfire Hero get destroyed in combat you can get one last push for damage by dealing damage to an opponent equal to Heartfire Heros power