Duplicate Files tab:
If the user alters the order of the column headers, DC should remember this. I like the Group header next to the File Name header much more than at the end behind the MD5 column. Now I must drag the header every time to view it when starting DC.
Version 2.0 will do this. Column order isn't saved in 1.x as the column ordering itself was a crash-prone feature that was removed for several versions.