There's basically no good outcome for your opponent when this thing hits the table
3 Uncommons There are 100 uncommons from Tarkir: Dragonstorm that can show up in these slots
is a daunting mana cost that basically restricts this blue creature to mono-blue or 2-color decks, but recasting your best instants and sorceries extends the lead cards like Time Warp and Breach the Multiverse naturally provide
There are some clear, high-value cards you might have heard of Mana Reflection, Dryad of the Ilysian Grove, and Dark Depths
Each instance of extort triggers independently, so you can pay two for two drains for each spell cast
We can't identify a ton of new cards that would make Rot Hulk loads better than it was before (though Geralf the Fleshwright seems like a fun one from MTG Thunder Junction), so it's likely the overall popularity of Wilhelt is increasing, rather than the proportion of new Wilhelt decks containing Rot Hulk going up