On the trip over, I scored a seat near the galley exit. Meaning no seats in front of me where the overwing exit door was. I could stretch my legs right out, and every hour or so, I could just unbuckle my seatbelt and stand up and do a series of stretches and sit back down again without bothering any other passengers or crew.
On the way back, the plane was nearly empty, so United allocated a full row of 3 seats to everyone in economy. Being able to lie flat was a massive bonus and I slept most of the 17 hours lying down and didn't feel it.
The 787 itself is a big factor in my comfort. The strength of the carbon fibre fuselage means that the cabin altitude is set to 5000 feet instead of the normal 8000 feet in other aircraft, and the air is recirculated more often I believe, which leads to less fatigue and high altitude problems.
The aircraft also cruises higher than most other earlier era jets, so we were usually well above most weather patterns for a very smooth ride. The new cabin lighting also helped to set the mood for sleep and awake cycles while on board. Kudos to Boeing engineers for the design work that went into this.