Everything is important. It doesn't matter what it is that makes it enjoyable. The point is that there has to be something that makes me want to use the headphones and at the same time I shouldn't be annoyed by something else too much.

But if by transience you mean dynamics I have to agree that it's often overlooked. A good example of this is Thieaudio Phantom which is a pretty terrible headphone overall but it does dynamics so good I still keep them as a benchmark. Another examples are DT900, HD660S, or K712. People often say that they're not worth it because of DT880, HD600/6XX, or K701/612/702, completely ignoring a huge gap in the dynamics. On the opposite, this is also the case with Hifiman's non-round open-backs which on the opposite lack the dynamics and people often ignore it, saying for example that Ananda is the straight-out upgrade from Sundara which isn't true.