As you've already read, there are a ton of features mentioned throughout this article. This area is to highlight a few that haven't come up, but we've found to be extremely important.
The closure on MMA shorts not only keep your shorts on, but can also cause discomfort or even damage to your shorts if not designed properly. Typically most MMA shorts will have a drawstring and Velcro hook and loop closure, with Vale Tudo, muay thai and compression shorts having no additional closure system. For MMA shorts, I prefer a closure that flaps over the top of the waistband vs from the side simply because it prevents any exposed Velcro from irritating my skin. There are also big differences in the quality of Velcro. Newer styles feel more like a Command Strip vs the traditional fuzzy/scratchy option. This means there is less of a chance of your scratchy Velcro rubbing on high-end material like Lycra during your training and essentially shredding it.
Side Slits or Panels
Training in any sport, whether BJJ, muay thai or even traditional boxing requires your legs to move freely. No matter what style you choose, you'll most likely have the option of side slits or panels that offer great freedom of movement. Traditionally, side slits, cut high into the sides of the shorts were needed. However, materials have come a long way and you easily can practice muay thai in the right pair of board shorts if they're built well.
Simple, right? Just tie a knot and let's go. Not so fast! Think about your favorite running shoes. Think about all your shoes. Different laces for different purposes. Same with your MMA fighting shorts. Make sure that drawstring is durable, and grips well when tied. The last thing you want is a waistband that breaks or comes untied at the worst moment.