The light. Beginning with the fact that I actually get to see the sun occasionally -- it's not pitch-black when I'm heading in to work in the morning, and there's still light by the time I leave in the evening.
But even more than that, there's a certain quality to the light around 5 o'clock these days that just says "spring" -- something about the angle, maybe. I can't quite put my finger on it, but it's a very spring-like light.
The temperature, of course, is not a reliable indicator of the proximity of Spring, as this entire winter season has taught us. But it's nice when it lines up with the light to give one the full spring-is-coming experience.