Using the Bookmarks extension 0.6.5.3 (or the built in Bookmarks manager), when you select a bookmark in a folder, that folder is closed when you go to the website you've selected. If you decide you then need another website in that same folder, you once again have to go through the very same steps in order to order to open the same folder. This process makes selecting bookmarks in folders very slow. Other browsers (Chrome, Firefox, Internet Explorer) have bookmark managers (either built in or extensions) which keep the folder you are using open thereby making selecting bookmarks very fast.
This is a feature I would like to suggest for Opera.