Wheels. I'm dieing to throw on a wheelset on mine, but I'm waiting until we get some nice Spring weather.
What I have coming in now are Planet X Ultralight CNC Brakes w/SwissStop Pads, Dura Ace rings to go on my Ultegra SL crankset and a set or Dura Ace 7900 Pedals. I'm also looking into black cable housings because the Dura Ace gray just stands out too much. If they were black, they'd disappear.
After that, the wheelset (Mavic Ksyrium Elite/Soul S2.0/BWW Blackset Race 11?) & an Edge 2.0 Fork.