On Windows, immediately adding Dropbox to 'Favourites', and the icon integration to show sync status is an awesome feature. It is invasive for Dropbox to hijack your icons and overlay a checkbox? Yup. But incredibly helpful in my opinion.
Maybe it isn't everyone's cup of tea, but the close integration with file browsers on multiple OSes is (was?) definitely one of the differentiating aspects of Dropbox, polarizing or not.