A selection of common UX problem areas and possible solutions.
1. Allowing users to shoot themselves in the foot by permanently deleting their data.
Disk-space is cheap, and user whims are fickle. If you can absorb the extra costs, I suggest never permanently deleting anything a user creates, unless they cancel their account. Give your user a history page where they can restore the state of everything they've ever created or uploaded with your app. When they screw up (which they invariable will), they'll love you for thinking about the issue.
2. Interrupting user interaction with unimportant modal dialog windows.
Instead of rudely throwing a modal dialog in your user's face, consider using Gmail's method for atomic operations: soft delete. After every action, Gmail displays a nonintrusive status message at the top of the screen. The message tells the user exactly what's occured–e.g., "This conversation has been archived."–along with a simple "Undo" link.
Why is soft delete better than modal confirmation windows? For one, this method allows the user to delete faster by skipping a step (the modal middleman). Modal windows are intrusive, workflow-breaking, and force users user to make immediate decisions. Inline undos are far more user-friendly because the entire web app stays usable, instead of being backgrounded. Let's face it–most deleted files are never wanted back.
3. Not allowing users to do batch operations with a list or grid of content.
Native file managers allow clicking and dragging to lasso multiple documents. We've used this simple operation on our desktop for years, but it has yet to show up online. Select-lassos could work very well for photo-oriented apps.
4. Forcing the use of lightboxes in image galleries.
Lightboxes break the UI conventions of the user's operating systems. The non-standard chrome tends to difficult to close and impossible to move. Solution: <a href=image.jpg> with a magnify icon. Let users decide if they want to open the image in a new window or not.