I made the mistake of buying a pair of side zip boots my first time out. The damn zipper ripped off in the middle of a nice February day. Made for a real comfortable rest of the day.
I've had good luck with Carolina boots, especially the ones with thinsulate in the winter time. They're incredibly warm. The only downside is the fact that the sole doesn't come out. So you can't replace it with a Dr. Sholls gel insert. Which I highly recommend by the way. Especially the ones specifically designed for work boots. It makes a HUGE difference.
The removable insole, and a defined heel (Required where I work, and I imagine most other places) are two major things for me. Comfort, and being able to lock yourself in while riding a car, or climbing ladders are top priorities obviously. If you don't have a defined heel it is very easy to slip and either smash your knees on some unforgiving iron, or fall right on your tail.
Another thing that is overlooked sometimes is a good pair of work gloves. I wear these gloves:
http://www.professionalequipment.com/ki ... er-gloves/
They are insulated so the wintertime your hands never get cold, and I even wear them in the summer. Your hands sweat only a tiny bit, but the added insulation keeps your hands comfortable when grabbing ladder rungs all day. Not to mention if you grab something sharp the leather is an added layer of defense.