When you train in Aikido and martial arts you want the best facilities available; an important consideration when training in a martial art where throwing your training partner down and learning to control them becomes a part of the training.
The facilities in the dojo should provide as much protection and comfort as possible in order to avoid unnecessary injuries while providing a training area that fits what you'll be doing.
In Japan, the very best dojo have what is referred to as a "sprung floor". That means that the training floor is placed on top of springs and on top of the springs is a layer of plywood, then Olympic-level mats to cushion the falling, then canvas so that you can move easily but not have your feet stick to the mats.
This type training surface is commonly referred to as a "fast floor" meaning that you can take a lot of falls without hurting yourself while still being able to walk and move quickly. It is a vastly superior training surface than simple folding mats laid over a concrete floor.
There are other items to look for that may seem commonsense but surprisingly not all dojo have them, such as separate dressing rooms for both male and female students, and separate rest rooms. And, it's nice if the dressing rooms have clothes hooks so that when you suit-up for training, you don't have to lay your clothes on the floor. So being able to change at the dojo (and not drive in traffic in your uniform) is always a plus.