I just completed IAD-ADD-BKK-ADD-IAD. Yes, a long time in the airplane. Thankfully, in business class (Cloud 9). A few things to add to the mix:

The stop in Lome (LFW). On the way to Ethiopia, we stayed on the plane. The cleaners came on and here was refueling. However, on the way to the United States, everyone had to disembark with all of our belongings. Thankfully it was a jetway and not a bus gate. We were kept in a cordoned-off part of the gate for LFW-IAD. There was no A/C and the temperature was 91F. I heard that one could use the restroom, but would have to come through security. Otherwise did not have to do security again and simply re-boarded. Nevertheless, it was an unpleasant part of the trip. The word was it was a requirement of the US Dept of Homeland Security. We also went through security at the gate in ADD before the flight. A small thing at the Lome airport, from being in the cordoned off area, I had to show my passport and boarding card to the gate agent, some official in the jetway, and the flight attendant, all within 100 feet of each other, for a group already approved for ADD-IAD.

Yellow Fever: Coming from the U.S., I didn't think Thailand would care about yellow fever and an airport connection in Ethiopia of only a few hours at which I did not even enter Ethiopia/leave the airport. That would be wrong. When I got to the immigration officer, who wants your passport, boarding card, and visa (if applicable), as soon as he saw Ethiopia, he handed it back to me and said I needed approval from the health department at the airport. I had to go there and fill out a form and get approved. As it happens, I have yellow fever vaccination so it wasn't a big deal. For those without, I don't know what would have happened.

Bangkok Airport: at checkin I was invited to use the Miracle Lounge. Well it is a cramped space, no restroom, and limited food options. Not recommended if you have alternatives. I have star gold status and went to the Turkish lounge, which was a mile better.

Service: I found it to be fine. The quality of food was ok and the quantity massive. I found the internet access to be spotty, probably the worst I've experienced - by contrast i've done Munich-Washington on United Airlines and don't believe there was any interruption in service.

At Washington Dulles: Ethiopian is not a member of TSA pre-check. So even though I have Global Entry, I had to use regular security. At the time I was there, the line was minimal, only 4 people to the document checker, so it didn't matter. But I suppose it could matter depending on your timing.