Hi Zlatan,
Congratulations! Great score
I disagree with Kevin: I think gui design and development is one of the most difficult things, because you have to design a user-friendly gui that is easy to adapt for future enhancements. When i was developing my gui there were so many things you have to take into account: mnemonics, limit number of chars entered in a text field or not, block invalid chars or not,...
And your score of 28/40 is one of the most popular scores you can get for gui, just like the mysterious 44/80 score (for locking), so your gui has to lack something important to my opinion.
Kind regards,
Roel