Although I like the "stacks" feature in the Bridge, as discussed in my last Photoshop post, it's not without flaws. Let's look at things I would classify as problems, or at least unintuitive.
First up, there are 2 operations, "ungroup from stack" and "expand all stacks" that are a little too similar. "Expand" makes all the stacked pictures visible like in the original list of thumbnails. "Ungroup" makes a stack disappear and all the pictures are separate again. If, in a moment of distraction, you select all pictures and do "ungroup", poof all your stacks are gone. I lost a couple of hours worth of tedious work this way, after stacking together zillions of triplets produced by auto-exposure bracketing.
But, you say, "Undo". Sorry, Edit->Undo doesn't work for stack or unstack operations.
The menu operations would be less likely to get confused, if they employed consistent terminology and structure. You've got "Ungroup from Stack" and "Open Stack" right next to each other. "Open" does the same as "Expand All", only for a single stack, so why not use the same verb in each case? To ungroup all stacks you must select all, there's no menu equivalent to "Ungroup All" like there is for "Expand All". So the menu operations are indistinct in terms of what verb is used, and whether you need to select all first.
I wouldn't have been futzing around with "Ungroup" except that the "Group" operation has a problem with keyboard shortcuts. If you select a bunch of separate pictures, Cmd-G groups them into a stack. Select multiple stacks, or some pictures plus another stack, and Cmd-G does nothing. It doesn't work if any of the selected items is already a stack. To do that via the keyboard, you have to ungroup the existing stacks, then add new thumbnails to the existing selection, then group again. Yet you can drag-and-drop separate pictures onto an existing stack, although only when the stack is closed, er, collapsed. So a mouse operation doesn't have a keyboard equivalent, even though it would make sense for Cmd-G to handle the case of merging or adding to a stack.
The online help says that selecting the top picture in a stack means that operations apply to all pictures in the stack. But that's only true if all the pictures in the stack are selected, which they are when you first group them, or if you click on the thin 3-D border around the right and bottom sides. So in experimenting with stacks, I ended up with several cases where labels or keywords intended for the whole stack were only applied to the topmost photo. Also, when you mouse over the bottom region of the border, a tooltip comes up that obscures the whole clickable bottom area, so in practice it's only the right part of the border that you can use for selecting the stack.
The Bridge can be sluggish to process input events. A number of times, I've done a Group or Open operation on a stack, selected other pictures and done Cmd-G to group them, and had the Bridge decide I really wanted to open those pictures in Photoshop. So even though stacks save time overall, while putting them together you have to pause between clicks to give the Bridge time to catch up.
One nice feature of stacks, related to the drag-and-drop idea, is that you can stack photos that aren't in sequential order. Have you ever taken several shots of some landmark, taken shots of something else, then more shots of the first subject? (Anyone who has photographed Half Dome in Yosemite knows what I'm talking about here.) With a stack, you can put all the pictures of that one subject, even from different days, into one pile where it's easy to pick out the best one. Or, once they're in a stack, you can give them the same keywords or label.