Doesn't matter what fantastic features it has, when a mouse doesn't fit YOUR hand or the way YOU grip the mouse, it's a thorn on the side.
Second most important aspect is scroll wheel. The wheel is the thing on the mouse you use the most often, it must not rub you the wrong way.
some mouse, it's hard to scroll. You'll feel it after a while when you scroll all day in a day job.
also watch out for mouses with slippery wheel surface and strong wheel resistance. With such mouse, it requires you to hold the mouse firmly, else you slip, or the mouse gets pushed.
for more about Logitech's spin wheel, see: Logitech Mouse with Spin Wheel
For mouse with more than 3 buttons, the button's position and feel must be right.
For 5 or more button mouses, the extra buttons don't have standardized positions. When the position isn't right, some button will be very hard to use, essentially making them useless.
If you are buying a mouse with more than 3 buttons, it's critical to check those extra button's position and feel.
3 buttons is basic. But 5 buttons is quite essential. The extra 2 buttons can be used for forward/backward browser page, or switch apps, switch tab, or close tab.
For gamer, or a programer, or digital media professional (web/game design, 3D modeling, video editing), you'll want 7 or more buttons. See: Why You Need a 9 Buttons Mouse?.
Most mouses are right-handed. 4th and 5th buttons are placed for your thumb, which is the best. If you are left-handed, you might shop for a dedicated left-handed mouse with dedicated shape. A left-handed mouse is much better than so-called ambidextrous (symmetric) ones.
Here's 2 left-hand mouses.
Laser lets you track more surfaces, such as on glass surface or carpet. However, laser does not mean better, especially for gaming. See: What's the Difference Between Optical Mouse and Laser Mouse?.
DPI is NOT important, even for gaming. Basically, all mouses you buy today have sufficient DPI even if you have huge screen. But also, you can adjust mouse pointer sensitivity in {Windows, Linux, Mac}.
DPI is a marketing ploy for gaming mouse. The importance of DPI is a undying myth. In fact, if you are a pro gamer in first-person shooter games, you probably should use a lower DPI (⁖ 800 or 1200) and turn on acceleration. Because lower DPI gives you precision, and acceleration gives you speed for fast turn-around. See: What's Mouse DPI? Does Mouse DPI Matter in Gaming?.
For serious computer user, gaming mouse is always better, because:
Gaming mouse often features colorful led light too, but it's not necessary.
There are many other aspects of mouse and features, but mostly it depends on person, need, preference, and choices should be obvious.
• Wired vs wireless. Wireless is convenient, but with wired mouse you don't have to worry about battery or charging. Wireless mouse sometimes have tracking problems, especially when you have lots other wireless devices, but this problem is rare in today.
• Mouses differ in weight. Wireless mouse often has battery, so it's heavier. Some gaming mouse has detachable weight, that lets you adjust. Kinda silly.
Standard 3-buttons mouses works in any OS without extra software.
Microsoft's mouses come with IntelliPoint software. It works for Windows ＆ Mac. (Microsoft Windows stopped making gaming mouses)
Logitech has software for Microsoft Windows. For their gaming mouse with extra buttons, you can use the Windows software to configure your buttons, then, those extra buttons can be used in Mac ＆ Linux.
If you want a fancy/gaming mouse with Linux driver (so you don't need to setup in Microsoft Windows first), a well known one is by the company ROCCAT.
