I'm pretty minimalist as far as specialized clothing. So for me it's t-shirt and shorts from 50 degrees on up. Below 50 I'll add long-fingered cycling gloves and a sweatshirt. I don't think about long pants until the temp drops below freezing.

Dress so you're chilly the first couple miles, then you'll be about right for the rest of the commute. If you start off toasty warm you'll be soaked in sweat by the time you get to work.