Your hands are by far the most vital part of any fighters body. With MMA this is even truer. Not only do MMA fighters use their hands for striking, but they also use them to grab their opponent and grapple them. MMA gloves are specially designed to allow you to grapple without bulky gloves getting in the way, whilst simultaneously providing adequate protection for striking.
Padding:
MMA Gloves have different levels of padding and style of protection. Some offer gel padding to absorb impacts, others use foam core. It is usual to see around an inch of padding across the knuckle area. They are not designed for heavy impacts such as bag work, rather they are designed for sparring or a tournament bout.
Finger-less Style Gloves:

A rigid boxing style glove simply will not work for an MMA fighter. You need the fingers tips free in order to pull off a lot of the MMA style submissions.
Quality:
You should look for an MMA glove made of high-quality materials. You want the gloves to last you for at least a few years of use. You should look for high-quality leather construction, strong stitching, and generous foam or gel cushioning. The cut of the glove is one of the most important things you should look for. Make sure the cut of the glove is going to suit your fighting style and hand shape.

Size:
This is where people often get stuck. Very often people ask "How tight should my MMA Gloves be?"
MMA Gloves should be snug on your hand but not restrictive. You do not want them so loose that they slide around whilst grappling, but equally, you do not want them to cut off your blood supply either. It is best to research the gloves in question, examine the sizing guide, and work out the correct glove size for your hand measurements.