I noticed that when cropping an image using the aspect ratio guidelines, there's a gray box with a 2x3 in it that either appears on the left edge of the image or the top, but never both. What exactly is that for and how is a user able to determine when they've set the proper aspect ratio?
The gray box is showing the area of the image that will be cropped when using that aspect ratio. You can place your cursor in the area that is NOT in the gray box and you can drag the image to adjust the area to crop. If you place your cursor outside of the image you can click and rotate the image if it needs to be straightened. When you do that you will probably see that there will be gray areas on either sides of the image as well. The fact that you see that gray box is an indication that the image is being cropped to the selected ratio. For example, this image was cropped to the 16:9 aspect ratio and had to be straigthened a bit. Notice that gray areas on all sides of the image.