Everyone who likes read about best practices in usability and user experience (UX) may remember that heuristic by Jakob Nielsen: Recognition than recall and this include make URLs more easy to recognize and remember.
On Information Architecture articles and books, good URLs are frequently associate with friendly URLs, like the URL of this blog posts instead of, as example, "http://www.myblog.com/post.php?id=1235652".
However, as more users are accessing websites from mobile devices, friendly URLs may not enough to represent good URLs by two main factors:
Input field to type URLs is too small. So, URL must short to be seen, so the user can check spell easily and correct it if needed;
Even with QWERTY keyboards and touch screen, type words in a small screen still needs more effort than a desktop/notebook...