Frankly, I always found Exalted's system to limit the amount of cool at my table, partly because it always felt more like a bribe to get people to do more then just roll dice then a reward for doing so.
Exalted also gives back Motes/Willpower, which is the real mechanical reason you want to stunt as a player.
I'd go with Exalted's tier system as laid out by the original poster. If you do anything besides say "I hit" you get a d4 to add to that roll. If you use the environment as well, you get a bonus action die fitting for your level that can be used on the roll or just saved. If the description is really, really cool, I might just give them an auto-Threat, plus an action die
The other important thing about stunts is that they let you do the impossible, which is a neat rule, but only if it fits your campaign. It's great fun in Exalted to say "as the archers unloose their arrows, I use them as stepping stones to climb my way up to the flying sorcerer" and have it happen. In a Game of Thrones campaign? Not so much