In Dolphin 2 you can add tags to files, but you can't add a column in the detailed view to display them. Is there a way to select files based on their tags ?
Yes. It is possible but you have to read the tutorial
See this from the manual Columns
as well as this Folders
The complete manual is HERE along with tutorials
Since KDE 4.1 it is possible to display the folders as a tree. In this mode a plus sign is shown next to the folder. By clicking on this sign the folder is expanded and all containing sub folders and files are also shown in the table but they are attenuated. By clicking the sign, which display a minus now, again the expansion is closed again. Of course the tree structure can be used for as many folders as wished. The tree view is disabled by default, but can be activated via Settings -> Configure Dolphin -> View Modes -> Details with option Expandable Folders.