Memory usage, file size, the fact that it tries to sign you up for utter crap when you first install, the fact that none of the original dev team still works for Nullsoft... last good Winamp was Winamp 2.95.
"Nullsoft was sold to America Online on June 1, 1999, and thereafter existed as a subsidiary of America Online. After the acquisition, Nullsoft headquarters were moved to San Francisco, California. Nullsoft subsequently released several new versions of the Winamp player, and has grown its monthly unique subscriber base from 33 million users to over 52 million users over the course of 2005. Nullsoft's San Francisco offices were closed in December 2003, with a near-concurrent departure of Justin Frankel and the original Winamp development team. Nullsoft is now a division of AOL Music."
Ever since, it's been crap.