The big problem with "one thing" is the extremely political matter of who gets to set a person's "one thing". Is it the employee? (Open allocation.) Or is it the manager? (Then it's dictatorial, and you'll have to offer 30-50% annual raises to keep people.)
Another problem is that it's a matter of framing, which is subjective. One guy, if asked to describe what one thing he did this year, might say, "One thing? Oh, god, I did zero things, because I was too busy doing a million things. I spent the first quarter putting out fires while we were scaling up the WhizBar, then I spent a few months looking over the call center's shoulder trying to figure out what they were doing to kill the DoodleBlat after we rolled out the Bilbo feature to production. At the same time we were debugging that thing that made flames shoot out of the Bilbo every Thursday night in the Central Time Zone during Daylight Savings Time. Then I was on that planning committee helping do traffic projections. And of course Q3 was when we did all the work to make the AcmeCorp deal actually work. Ask me about Q4 when it's over because frankly right now I don't even know what I'm doing this afternoon."
The same guy with a different manner of presenting himself might say, "I kept the servers alive all year, asshole. Now get out of my way or they're gonna die while I'm talking to you."
Another problem is that it's a matter of framing, which is subjective. One guy, if asked to describe what one thing he did this year, might say, "One thing? Oh, god, I did zero things, because I was too busy doing a million things. I spent the first quarter putting out fires while we were scaling up the WhizBar, then I spent a few months looking over the call center's shoulder trying to figure out what they were doing to kill the DoodleBlat after we rolled out the Bilbo feature to production. At the same time we were debugging that thing that made flames shoot out of the Bilbo every Thursday night in the Central Time Zone during Daylight Savings Time. Then I was on that planning committee helping do traffic projections. And of course Q3 was when we did all the work to make the AcmeCorp deal actually work. Ask me about Q4 when it's over because frankly right now I don't even know what I'm doing this afternoon."
The same guy with a different manner of presenting himself might say, "I kept the servers alive all year, asshole. Now get out of my way or they're gonna die while I'm talking to you."