a. It's not so HOT
b. It's so much more colorful. There are so many shades of red, orange, yellow, dark green, and even brown in Fall. Summer is just an overabundance of green.
a. It's cool, but not so cold that I don't want to be outside for long periods of time. It still gets warm in the afternoon and I don't need a bulky winter jacket.
b. In winter, everything is brown and the sky is always hazy.
I'll admit, spring is my second favorite season, but fall is still slightly better, if only because I think the colors are richer and everything looks more defined. Plus, it doesn't rain as often (with this fall being the exception, I suppose).