Gorm, since I love you, I'd say the main problem is Gyarados. It's on nearly every team and is hard to stop due to bulky defences/Taunt. He should be priority.
Secondly, I'd say that a Rapid Spinner is more important than ever. There are teams that aim to just wall you to death using Spikes, Toxic Spikes and Stealth Rock. Of course, in order to spin you need to defeat Dusknoir who is basically Dusclops v1.5. Pack something to beat him.
SpecsMence is a particular pain if you're not prepared, as can Scarf/Band Heracross. It's important to find out what item those two have so it's important to have somebody who can scout (ie Protect/Sub).
Sleep is even more prevalent, with Breloom getting better and Tangrowth, Bronzong, Crobat and Gengar running around. ALWAYS have a Sleep Absorber.
Blissey aint changed much from ADV, so I'm sure you know what to do there. Garchomp is real terror, but is often predictable so go to your bulky water. Despite the brilliance of Chain Chomp, it is very rare. Scarf chomp is common though.
Finally, although it isn't really a true-OU threat, Typhlosion with Choice Scarf is incredibly common and spams Eruption. Can be a real twat.