I'm in the process of (slowly) shopping for a new display case. Some of the criteria that I have looked at include;
1) Sliding front door - this allows good access to add/revise displays, but keeps out the door frames and mullions on some cases. Not cheap, but the one clear glass panel allows maximum viewing. I learned about this from my son's Code 3 firethruck collection.
2) Either multiple lights or the ability to add more lights. In the case I have now, the lights are in the top and the middle shelves, the other displays sit in the shadow of the upper displays. This will be more important when I add ground cover to the diorama scenes.
I also agree about the desireability to be able to add some more shelves, just remember that the display will be more visually appealing if you don't "cram" too much stuff together.
I like the idea of a display case as it reduces (but doesn't fully eliminate) dust on the collection. I also like the stuff kept back from the little inquisitive fingers of my grandchildren.
Gary