Fork used to have a very negative connotation, and that was related to the difficulty required to pull one off. When you forked a project, you had to set up your own hosting, web space, issue tracker, version control system (and you probably couldn't clone and start off the upstream's VCS), and so forth. The technical barrier made it so that you only forked a project when you have severe disagreements with upstream; some of the most famous in history include EGCS from GCC, and XEmacs from Emacs.

With DVCSes, including Git, much of that technical difficulty is automatically removed. Every clone can potentially be a fork (as in, a new independent project), and this was one of Linus's intentions. If forking is easy, it keeps upstream on their toes. After being adopted by DVCS hosting sites like GitHub, it turned around to being a positive term even.