An office seat can be either a throne from whence you command your subjects to work, or an electric chair to which your last days are committed. What's immediately obvious is that when it comes to chairs, one size does not fit all. Since the optimal height for a seat is one quarter of a user's height, it stands to reason that a chair that does not have a gas lift for height adjustment is going to be a plague to more people than it is a benefit.
We've all used chairs that are height adjustable, but that is really a bare minimum requirement when looking for a chair that will contribute towards your ergonomic well being. The backrest should be height adjustable and also tilt adjustable, so that it can properly support your back. Ideally, a chair should also have armrests to prevent you having to hold your arms above your centre of gravity for prolong periods and, in the best case scenario, those arm rests should be adjustable too. The aim is to keep your arms at desk height so that you don't have to hunch your shoulders. Your elbows should keep a nice 90 degree angle, assuming your back is straight.
Don't forget your lower back, either. If your seat back is adjusted to support your middle and upper back, you'll need a lumbar support to keep your spine in check. Something as rudimentary as a cushion can do the job, but a chair with individual adjustments for lumbar and height is rather preferable.
Stability is an often over-looked area of chair ergonomics that can contribute greatly to your happiness or otherwise. Although it is undoubtedly fun to scoot around on wheels (racing optional) or spin idly while considering the finer points of some complex calculation (dizziness optional), consideration should be given to whether or not that functionality is really necessary. Sitting in a stationary chair is rather easier than having your body's core constantly re-aligning and so a chair with legs, rather than wheels, is generally better for you. If you really have to have wheels, always get a five-pronged base, which is more stable. Also bear in mind the surface your chair is going to be on - if you're going to have wheels, pick ones which match the type of floor. Wheels which scoot well on carpet might not be so good on wood flooring, which can cause unnecessary tension.
Moving down, give some thought to your thighs. Constantly pressing against the edge of a chair can be bad for your muscles, so look for a chair with a ‘waterfall front', which tapers away to allow your legs to sit comfortably.