On data import currently there is a bar running back and forth for user entertainment. If the number of rows already imported would be shown too it would give a good idea how long it is going to take. (Updating the number once a second should be enough.)
I am a number person, but some people apparently prefer a real progress bar (going from 0% to 100%). This could be done by calculating the ratio of how many bytes were already read and the total file size. (I do not want something like "rows imported/total rows", as to get the total rows the whole file would have to be read first.)
I don't know how the data rows are read, but if the source file has either one byte for the next line (Unix/Mac?) or two bytes (Windows) the bar can be slightly off depending how it is implemented (not reaching 100% or overshoot). A Windows generated file should not cause problems importing under Unix and vice versa.