The obvious throwback to the OT was the Tantive IV and Captain Antilles. My personal fave was Tarkin. I had no IDEA that a single shot of an actor with no lines or closeups could convince you about who he was and what he was doing just by his look and his posture. The guy should get an Oscar for Best Walk-On.
Other throwbacks in no particular order:
"S-foils" line in the opening battle
Yoda clutching his heart, sensing a great disturbance in the Force
The mere use of escape pods
And the throwback that should have been: A Jedi ghost