By default the filter gallery window is divided into three sections, with the preview on the left, the filter thumbnails in the middle, and settings on the right. Below the settings area is the stack--a list of all the applied filters. Here you can change the order that the effects are applied, or click the eye icon to hide a filter effect. By default, clicking on a new effect changes the top layer in the stack. To add a new layer, click the button at the bottom of the list, or Alt/Option click a filter thumbnail. By stacking multiple effects, rearranging the order, and toggling effects on and off, it is now much easier to experiment and find those happy accidents.
Right below the OK and Cancel buttons, there is a menu listing all the filters in alphabetical order. How many times have you known which filter you wanted, but not been able to recall which submenu category it was in? Now you can use the menu to go right to the filter you want. In fact, I prefer to hide the filter thumbnails and just use the menu which gives me more room for the preview. The double arrow icon to the right of the thumbnails will toggle them on or off.
Try this combination to give an image a bold, graphic look as you experiment with the Filter Gallery:
Top Layer: Smudge Stick
Bottom layer: Poster Edges
Turn the visibility on and off for each layer and note the difference. Now reverse the stacking order (using drag & drop), note the difference and revert it back. Try adding one more effect to the top of the stack
Once again, experiment with the stacking order and visibility of the effects.