Have you ever drawn Tendrils of Agony in your opening hand? It's miserable - you can compare it to drawing multiple copies of Chrome Mox. For this reason, Tendrils of Agony has been moved to the sideboard as its usefulness in the main deck was decreasing. What caused this change is that I found that most of the time that I was using Tendrils of Agony, that I had more than enough available mana to Burning Wish before casting it. This isn't anything new (the change happened in 2012), so those of you who are experienced with The EPIC Storm shouldn't see this as a shock. That said, there are times when sideboarding into Tendrils of Agony is correct, typically these are match-ups with an overload of mass removal for our goblin tokens. The other time siding in Tendrils of Agony is what you want to be doing is against a deck like Burn where Ad Nauseam isn't effective.