A lot of runners sing the praises of fall because they welcome the cooler temps for their runs.
Myself? Not so much.
Don't get me wrong; I do enjoy fall and the cooler temps can be pleasant. The cooler temps, though, can also be unpredictable and extreme (ummm, low-40's in the morning, high-70's in the afternoon, thank you very much).
Let's not forget the shorter days also throw in another nuance...limited daylight. My early morning runs (and occasional walks and bike rides) are now in 100% darkness. I'm still getting them done, but the change of seasons dictates I change a few habits for the time-being.