So some thoughts.
I think this makes sense for Houston. Celtics fans have become spoiled with our player quality at the wing, but Houston is pretty limited in that area. They are betting that they can A) Be better playing small ball with optimum spacing on offense and switching everything on defense and/or B) Pick up a cheap big on the buyout market who can be useful. Even if Capella is 100% healthy that strategy makes sense, add in the fact that he might not be and if makes a lot of sense. It also opens up about 5.8 million under the tax line which could allow them to expand this trade even further to take on another rotation guy. Markeiff Morris makes 3.2 million, Aaron Baynes 5.4 million would both be options.
Atlanta is betting that they can't do better this offseason with free agency. They haven't traditionally been a city that draws free agents and Capella is better than most of the free agent centers on the market, with the possible exception of Drummond. And he is cheaper then Drummond will be long term. The team is awful defensively, so getting a better than average defensive center is big for them. All they gave up was the 15th or 16th pick in a bad draft year when they already have a likely top 5 pick and a ton of young guys. makes sense for them.
Nuggets didn't want to pay Juancho or Malik, so they turned those guys into a cost controlled asset in the HOU 1st. If you assume both those guys would leaving and given their depth, it makes some sense.
Wolves get a couple RFA's who might get squeezed by a tight free agent money situation. They get a decent first, makes sense for a rebuilding team.
So all in all makes sense for everybody, not a home run for anybody but defensible.