The ease of use, it's ability to be used by non-technical users without any training required (even for those with no computer experience).
I dislike that you can't save your work after closing out an account or project - but this may just not have been implemented yet in their latest version which was released last month! It has made our job much easier as we are able now create accurate drawings from customer provided plans instead having them send us pictures over email/phone calls each time they make changes etc., saving both money & resources!!