Savannah Lions, for the longest time, was among the most powerful one-drops ever, as a vanilla 2/1 for was utterly wild value so much so that watered-down versions in other colors with profound drawbacks were printed time and again (Mtenda Lion, Jackal Pup, Goblin Patrol, Jungle Lion, Scarred Puma, etc.)
Firing it off at instant speed is also important, especially for targeted removal
Looking at your curve again, youre able to get both of these down as early as turn three, and then its off to the races from there
Also, the mana dorks make Winternight Stories better as ramp to it and discard fodder for it
The Viper is too slow for more aggressive decks which tend to prefer their card advantage to come with casting or killing the monster rather than the effects of the monster when active in play