After using several miter saws, I realized that the most important thing is how accurate the saw can cut. If the saw doesn’t cut where you want it too, consistently, then the saw is useless. Although most new saws will cut accurately, a lot of older saws tend to get warped, resulting in inaccurate cuts. So it is important to get a new one every so often.
Another feature that I have realized to be important is what angles can the saw cut. Some saws allow you to cut at various angles, while others only offer a limited amount of angles.
Make sure you buy a saw that gives you the freedom to cut every angle possible, whether its 90-degree cuts or bevel cuts. The more that saw allows you to cut, the better.
The last thing that I look for in miter saws is how easily is it adjustable. When I use a miter saw, I want to change the angle of the saw easily, both saving time and frustration. If adjusting the saw becomes a hassle, I will get irritated.