Whenever someone complains about a new version of a bit of software, my general response is "you don't have to upgrade".
For example, complaints about Vista can generally be countered with "Well, Windows XP still works fine.."
Where this argument entirely falls down is when an upgrade is forced, but there is pretty uncommon (online portions of video-games, instant messaging applications)..
..that is, until you enter the land of web-apps..
With a web application, or web service, or website, users have absolutely no recourse - you can't install an old version of del.icio.us.
The only webapp I am aware of that allows you to use a previous version of an application is GMail, where you can select "Old Version".
The problem is, it's basically twice the work. You now have to maintain two applications. You can't make changes to the database that would break the old version (without changing both code-bases).. Is it worth it, to keep users who might otherwise have gotten annoyed at an upgrade and migrated to a competing site?