Range of movement, coverage and support where you need it. I turn my body every which way, including upside down, in a room that is 95 degrees. A regular shirt will not work for me. But it doesn't have to be yoga specific per se. Running tanks can work too.
That sounds like my idea of torture! But then, I wilt if it gets above about 85.
The few yoga classes I took, the instructor just told us to show up in whatever's comfortable, so I have no clue.