Unlike many other themes, it has a decent amount of documentation (check the sidebar for a full set of links).
It's got an extensive API (in addition to the standard WordPress one), is child theme-friendly, and is used by thousands (the forked WP.com version is used, for example, by Terence Tao). Many wonderful users have contributed translations in over 20 languages.
I don't really have a huge amount of time to improve or maintain it these days, although I do try to keep it up to date with the changes that come out in new versions of WordPress. If anyone is interested in contributing or taking over as maintainer, please do email me at firstname.lastname@example.org. The code is, of course, up on GitHub.
I haven't looked at the source in at least 2 years so I'm hoping it's not still using the same anti-patterns it originally used.
Downvoting alone doesn't do the job.
A community writeup on which frameworks and parent themes we really use and why.
Featured question on the stack exchange (beta) Wordpress site http://wordpress.stackexchange.com
I for one have always found it painful to find a decent wordpress master theme/framework to work off.