Cleaning: If you do more than just occasional snacking at your desk, make sure any mouse pad you choose is easy to clean.
The number of RGB zones (not the same as the number of LEDs) will determine how complex (or not) the light show gets. This doesn't necessarily need to plug into your PC, unless you also want to customize the mouse pad's RGB using its companion software.
RGB: RGB mouse pads are pretty, but they're powered via USB.
You definitely don't want to run out of room when you're lining up a headshot. Do you need a gigantic mouse pad? If you play games that require extreme precision and control - such as first-person shooters - you might want to consider a larger pad precision is generally attained with a lower DPI, and a lower DPI means bigger hand/arm movements.
Size: Mouse pads start small but come in a handful of sizes - the largest ones will take up your entire desk.
Alternatively, soft cloth mouse pads generally offer more precise control. These are best reserved for more competitive gamers (or at least players with a mouse featuring DPI control).
Material: Hard, plastic surfaces are more slippery, so your mouse will glide rapidly with little force from your hand.