In reply to mark k..

Because changesets provide the technical architecture to test, store and schedule changes without affecting the 'live' site. That's hugely significant.

Tell me what is the more mature, user-friendly way for users to edit their sites:

1) Toggling between back-end and front-end and hoping changes will work out nicely but not knowing in advance and having web visitors be exposed to your experiments.

2) Live preview controls so that you no longer have to switch between back-end settings screens and the front of the site, and you instantly see what a setting change does.

3) Test, design, experiment, tweak to your hearts content without affecting the live site. Compare different sets of changes and share those changes with others for feedback. Take those changes live when you're ready.

Today, 3 is only really possible if you setup a staging and/or development site. This puts it out of reach of basic site owners while introducing user-hostile complexity for the rest. Changesets helps provide a quantum leap for the site creation experience, making WP a much better platform to work with.