Played 6 games today and realized that a few things had to change:
Thoughtseize was cool and all, but losing 2 life is rough on this deck because of all the fetch/shock lands and due to the lack of any life gain. I liked being able to remove threats from my opponent's hand, but losing 2 life made it unappealing to cast sometimes. I've replaced Thoughtseize with Inquisition of Kozilek.
Creeping Tar Pit entering the battlefield tapped is a huge problem for this deck. It made me sad to have it in my hand and I kept putting it on the bottom of my library every time I saw it while scrying. Plus, it costs 3 mana to transform it into a creature, and with this deck, I'd rather be casting as many spells as I can each turn, so trying to keep 3 mana around is too much to ask. I've also tweaked the mana base a little to add more R/U dual lands.
One of the reasons I wanted to go U/R/B was to cast Terminate, but too many times it sat in my hand and I was unable to cast it because I wouldn't have a R or B mana source. Usually I either had R or B, but not both. Therefore, I've replaced it with the slightly worse Smother, and added an additional black mana source to help me cast it more reliably. Terminate may find its way back into the deck in the future when/if I add more copies of Polluted Delta.
Finally, I swapped out Sedraxis Specter for Young Pyromancer. The tokens it generates are critical to stabilizing/winning, and although Sedraxis Specter is a great creature, again, the way to win is to cast as many spells as possible, and 3 CMC is too expensive. It might have a place in the sideboard against some matchups, but for now I've cut it altogether.
