Good Tumblr user names are interesting, easy to remember, easy to spell, describe the content of the blog and have personal meaning. It is possible that a first username attempt will fail because all Tumblr usernames must be unique, and there are already millions of Tumblr users.
Easy to remember and spell usernames are useful because it is easier for an audience to find what they are looking for if the URL sticks in their minds. Descriptive usernames are helpful since many Tumblr users find new blogs through reblogged posts. If the username sounds interesting, these users may become new followers.
It is also possible to change a Tumblr username and URL if a better one is found.