I'd also point out the following:
In black,
Inquisition of Kozilek hitting every thing but Titan and Summoning Trap can really stunt their early development;
Memoricide, and Sadistic Sacrament (for the mono-black player)
In blue,
Out of the board, Spell Pierce can be a beating, especially on the draw (hits turn to Khalni Heart Expedition, or turn three ramp spell, as well as Green Sun's Zenith, if the Valakut pilot doesn't board it out.
In red,
Goblin Ruinblaster is currently a bit underplayed, but can still get in there. Tunnel Ignus can be an annoyance, though he is frequently lightning bolted before he does anything useful. I've also entertained the thought of boarding Destructive Force in RUG, but I don't currently believe the idea has any merit.
In green,
Not much to add here, unless you can easily cast Pelakka Wurm (looking at you, super-secret tech Eldrazi Green, which has been spotted in the Atlanta 5k, and at Grand Prix: Dallas). If you're boarding Nature's Claim, its also somewhat worthwhile to bring in, as long as it doesn't disrupt your overall strategy, and you really don't have anything better to do.
In white,
Also currently underplayed, Rest for the Weary wrecks the Valakut deck. Really hard for them to get around frequent 8 point life swings. In the same mold, Baneslayer Angel ruins their day. Kor Firewalker also serves as a good answer to Inferno Titan.
