Getting a decent mouse has always been a challenging decision to me. While most people can just settle for whatever is cheapest and available, I do have to account for a few idiosyncrasies:
First of all, size matters, and I'm a size queen. If you haven't been shopping for a mouse lately, here's the news: they have either shrank to the size of a nut for the sake of portability, or got ludicrously flashy lights to go with their overpriced tag.
On top of that, I have larger than average hands, and small mouses can easily become painful to maneuver after a few hours.
With the industry transitioning to all plastic, mouses have become increasingly lighter and inappropriate for my heavy hands. I have actually destroyed a couple mouses by accidentally driving them to the floor. Wireless mouses tend to be better in this aspect, since they have to carry metallic batteries. I don't like wireless stuff, however.
As mouses have been decreasing in size, they have also been decreasing in height, another painful trend for my hand. I have a claw-like grip that currently requires my wrist to rest on a palm rest attached to my mousepad, otherwise I'd have to spend over a hundred dollars on a decently tall mouse. It greatly diminishes my hand mobility, but I've been told by my doctor that's a better long-term solution than constantly moving it around the table.
Finally, what grinds my gears in most mouses nowadays is that they don't seem to be designed for human beings, let alone adult human beings. Most humans are either left-handed or right-handed. Therefore, a mouse should never be designed to suit both hands at the same time. A symmetric mouse is a flawed design by default.
Another crucial aspect is the number of buttons: a decent mouse requires at least 5 buttons (left, right, middle, back, forward), and a scroll. I don't mind if the scroll also dubs as the middle button, although this design has quite a shorter durability. If it is able to change DPI on the fly, then the DPI switch should be on top, not on the bottom. I'm not going to roll the mouse over in the middle of the game just to switch the DPI, especially if I paid extra for it.
Finally, construction is actually a point that has been improving over the years, especially in the gamer niche. Rubber is the main culprit: it's soft to the touch, but resilient to force, especially friction.
A brief history of mouses I have purchased