I like a forced air. You can take the temperature anywhere in there so that makes it easier. With a still air or thermal air, since hot air rises, it is very important where you take the temperature. That makes it a little more complicated.
An automatic egg turner frees you up a lot. You don’t have to be there several times a day each and every day to turn them, and you don’t have to remember to go do it. To me that is a requirement.
A decent sized window to look in. You need to be able to see what is going on in there especially during hatch.
A fairly easy way to adjust the temperature. I have an old model Hovabator 1588 with the dipswitches to adjust temperature. A total pain. You need some type of dial or easy temperature adjustment.