Windows 10 Mobile brings at least one big notable change to the table. One of the first things you'll notice is that the Store has dropped its beta tag and is now the default Store, replacing the old Windows Phone Store. You'll also notice a larger battery icon, which is a great change compared to previous builds.
Overall, however, this build is much more about speed and polish. It feels much smoother than previous builds, though there are some areas that still feel a little buggy. If you're on the slow ring and want to grab build 10166, you should be able to pull it down now.
Source: Gabriel Aul (Twitter)