The location needs to be convenient, and so do the hours.
I would love to find a photography studio where I could take my dog so he and I could be photographed together - which would, I think, necessitate a door directly to the outside.
A variety of backdrops and props would be really nice.
The one truly bad experience I had involved a coupon/package deal; the studio stuck to the letter of the deal, but there were quite a few additional charges - the cost per picture was much higher even if ordered at the same time, the pictures were stamped with the studio's name on the front (which I realize was intended to prevent copying, but it looked pretty tacky - gold Old English script just didn't go with the portraits I had taken), and the negatives were not available at any price... which wouldn't have been much of a problem, except that a year later when I went back for more prints, they'd gone out of business, and I couldn't reach anyone who had worked there.