You shouldn't read too much into Google Chrome's version numbers. Just because you can now install Google Chrome 3.0, it doesn't mean that Google wants to appear more mature than it already is. For Chrome, version numbers are just a way to highlight major milestones.
Three months after the first developer preview, Google Chrome 3.0 is out of beta and ready to replace the current stable version. Since Google updates the browser automatically, you might not even notice that you use a version that brings new features.
The new release supports themes so you can customize the browser with one of the 28 new themes. Not all of them are good-looking, but they're easy to install and you don't have to restart your browser after changing the theme.
Google Chrome 3.0 has an updated new tab page that lets you customize the pages reordering them using drag and drop, by pinning the pages you use frequently and removing the pages you no longer visit. Google simplified the page by removing the list of search boxes and the recent bookmarks.
(Tip: you can still use the previous new tab page design, by appending this flag to a Chrome shortcut: --old-new-tab-page.)
A feature you won't probably use too often, at least for now, is the support for the HTML5 video and audio tags. Like Firefox 3.5, Chrome includes video codecs that allow you to embed videos without using slow and unreliable plug-ins like Adobe Flash. You can test this feature in TinyVid.com, an experimental Ogg video uploading site, or in YouTube's HTML5 demo page, which uses an H.264 video.
One year after the first release, the numbers are impressive: "51 developer, 21 beta and 15 stable updates and 3,505 bugfixes". Google Chrome's market share is 2.84%, according to Net Applications, but the browser's impact was even more significant: Chrome set a high standard for browsers by focusing on speed, a simplified user interface and by handling web pages as if they were applications. Safari 4, as well as the the next versions of Firefox, are influenced by Google Chrome's simplicity.
In other Chrome news, the documentation for creating extensions is now available and the support for extensions is enabled by default in the dev channel. If you use the stable version of Chrome, you need to wait a little bit.