Abu Dhabi airport allows American citizens to clear Immigration before they get to the United States. In theory, it's a convenient way for American travelers to deal with security before arriving in the US. In reality, it has a couple drawbacks. First, the Eithad Premium Lounge is located before Pre-Clearance, a factor that must be taken into account before having too many glasses of champagne at the lounge.
If you're a points person, chances are you will find yourself in Abu Dhabi, my 76th 'country', for one night while to catch your flight to somewhere else. I've never had a desire to visit Abu Dhabi and opted to spend a month in Dubai instead. From what I saw in my short time there, Abu Dhabi is beautiful and may require a more extended visit.