WordPress released their version 4.0 last week. You can read the details of the new release here.
Usually when software vendors release a new version ending in dot zero, it signifies a major update. In this case it really should be called 3.10. The new version has some nice features that make editing and managing media easier, but no huge system-wide changes that will require you to retrain your staff.
Personally, I think this is good. As WordPress has evolved into a professional caliber business tool, stability becomes more important that chasing every new fad and feature.
This release looks solid. We recommend updating for our clients. As always, perform a full backup of your site and your data before upgrading. If you have automatic updates enabled, you may still need to trigger this update manually since it is technically a major release.