We also knew we needed to repair a leak from the stall shower. We weren't sure if it was coming from the door track, or from the drain itself, but the floor right in front of the shower door was so soft, under the tile, that you could poke a screwdriver through it! After also deciding that this would be a good time to remove the old wallpapered wallboard that was there, we started examining the shower stall.
We were going to have to remove the shower pan to get to the wood beneath in order to replace it, but were hoping not to have to redo the whole subfloor. In order to remove the pan, we had to start taking tile off the walls, starting with the lowest course of tile and moving up till we had enough room to prop up the pan and see underneath. As we did that, we discovered that the wall opposite the shower head had some grout missing at about shoulder-height, and that the water had been getting in there.
Now, the shower hadn't been used for about 6 years because we knew there was this leak, but we could see at the bottom, where we'd removed tile already, that there was a mold problem. So all we could do was keep going up to see how high it went. Long story short, we needed to remove all the tile and tile backer in the shower.
So basically the whole bathroom has been gutted, and we're starting over. The first improvement to the bathroom was actually adding a window to the room, when we were replacing the other windows on the front of the house. So at least we had some light and fresh air in there now!