Chairs are the worst. I hate them.
That being said, don't go for the mesh chairs if they're cheap. I had one I bought from walmart, but the band that kept the mesh in place was poorly installed with cheap glue and a few staples that quickly fell apart and ended up causing me more pain in the long run as the mesh slacked and my back dug into the top section of the framing.
Like Brandon mentioned, I too have a long torso, and I like to have something to sit back and rest my head on, but many of the ergonomic chairs only go up to mid back, otherwise they would be ideal.
Second to last issue that always bothers me is how the part for your button is lower than the part at the front edge of most chairs, so chairs that let you adjust the title angle is ideal as well, otherwise I feel like my hamstrings right before my knees get a lot more pressure than is comfortable.
Last issue which I look for in a chair is whether the height can be appropriately high. I'm 6'2", so when I want to rest my feet flat on the floor, it's uncomfortable for the opposite reason as the title angle; namely it puts too much pressure on my butt, while the front of my hamstrings are elevated off the set slightly.
Oh, and one more thing, slightly unrelated to the chair, but still sort of related is, can I have the chair at maximum height and still slide it easily under the desk? Meaning, do the arm rests get in the way of this process. Having arm rests that are only attached to the chair at the rear is ideal, as it's nice to have the desk low near my lap, while the arm rests can clear the top of the desk. Both an adjustable desk and proper arm rests is important for this.
One super very absolute last thing. How far out are the arm rests from the side of your body? If the arm rests are too far out, but you still want to rest your arms while typing on a keyboard, and the arm rests are not adjustable, then by necessity you have to put your shoulder rotation in a bad position for your rotator cuff muscles. In the long term that could lead to injury from something as benign as long hours of desk work.