Also, at least when compared to Windows (where everyone and his brother seems to feel the need for a custom and flashy toolkit, even within Microsoft), Linux apps have mostly standardized on look and feel. Very few people seem to feel the need to reinvent the GUI toolkit and make something that looks completely out of place on Linux (except for the proprietary vendors, like Google, who seem to be ignoring the established human interface guidelines and completely inventing their own look for Chrome)
Finally, as a developer, being able to delve into the source of libraries when debugging is invaluable.