(Though Ozai, the Phoenix King does this even better, allowing you to keep the excess mana.) Miirym, Sentinel Wyrm is solid, but only in green-blue-red dragon decks and nowhere else, so that's a tad limiting
Once you've announced that you're casting a spell, players can't take actions until you've finished doing so
Modern Madness Exciting as that is, this cards potential goes far beyond Standard
With a whopping 141 possible Commander options introduced in the set, you can expect exceptional variety and replayability
Finally, cards like Illusionist's Bracers and Sting, the Glinting Dagger make Krenko's activated ability easier to use, creating an astonishing number of Goblin tokens in just one turn
Repeatable card draw is kinda great, though, and pairing that with slowing down incoming damage is surprisingly playable