Actually the list is so long I don’t know where to begin! for a full list of improvements read the list on this link.
The new interface looks remapped, and I like how it can handle remote files like the files saved on Google drive, there has been work all over, I can see that.
One of the big changes I noticed was the styles menu, which you had to navigate to under format and styling in older versions.
Adding sheet menu to calc with related functions, and image control in impress, with the redesign of master sheets.
Did you install LibreOffice 5.1? Did you like what you saw?
*Update: I had to delete the configuration file of the old versions to fix some old issues like the webpage wizard crash, and to see the changes in looks.