One other item Jos reported was that the keyboard shortcut Insert source segment content doesn't overwrite the content of the target segment (instead it adds the content of the source to the current content of the target). I told him that this is probably by design.
The question is: is this the most handy/intuitive behaviour a new user could expect?
Yes, it is so by design. You can also select a part of source text and insert it in the target segment without overwriting it. You can overwrite when you select the part to be overwritten in the target segment.