There's a couple answers for this.
First off, you need to get your hands around a variety of mice out there so you can determine which one(s) best fit your hand. Remember, if you're going to be using this thing a lot, you want to be as comfortable as possible.
Secondly, you also need to consider the type of optics used by the mouse. There's two basic types: "optical" and "laser". The laser optics are relatively new and offer the following improvements over traditional "optical" mice:
Higher Resolution
Greater Tracking Stability
Greater Surface Compatibility
Several months ago I decided I wanted something different. I had (I still own it, but it's moved to a different Mac) a Microsoft Laser Mouse 6000. It works, but I was really never totally satisfied with it. A good portion of my motivation for replacing it (besides wanting to get Microsoft junk in general off of my desk) is that the scroll wheel on it doesn't have side-scroll capabilities (!!!), but only a single rightward-clicking app switcher function.
So, I actually went through two other mice -- the first was an Apple (wired) Mighty Mouse, and the second was a Kensington mouse. I can't even begin to express how dissatisfied I was with Apple's offering. It tracked mediocre at best, and would jump around the screen at random (which is a symptom of first generation "optical" mice of 6-7 years ago). And, of course, since it's optical, you can't use it on a number of surfaces, amongst them "wood grain" which is what the keyboard shelf on my computer desk is made of.
The Kensington didn't pull the same crap that the Apple mouse did, but it's tracking was so sloppy -- even after significant tweaking -- that I found it completely unusable. And since it, too, is "optical" I would have been stuck using a mouse pad with it.
I then, at the suggestion of a knowledgable CompUSA rep, bought a Logitech MX400 Laser mouse. The difference between it and everything that came before was amazing. It tracks tight and true, even without any tweaking on my part, it's scroll wheel side scrolls, and since it's a "laser"-type, it tracks perfectly well on almost everything, including wood grain and speckled textured plastic (which is what my other smaller computer desk is made from).
In short, at a minimum you need to get a "laser" mouse. What you get for features beyond that is entirely up to you. When you look at mice with more than two buttons and a scroll wheel (and a lot of times even then), the software they ship with gives you considerable ability to assign (or re-assign) shortcuts and functions to each control, thereby letting you customize and tailor the mouse to your own needs. Me, for instance, I never (or rarely ever) use Exposé, so I don't have it configured to trigger that function.
Hope this helps.