There are loads of enhancements that you can see a basic description of and hop into the tickets on Trac where they were hashed out. I'll be covering the new features in much more detail with the official release.
Importantly, if you are a developer, there may be some changes to consider for your themes and plugins, and Andrew Nacin has shared some of the things to especially be on the lookout for. Don't wait until 3.9 is officially released at the end of the month.
I'm always impressed with new WordPress releases, but the updates in 3.9 really polish a bunch of long-standing pain points in WordPress, and I'm looking forward to it. Also, if you're on WordPress.com, you'll see some things from WordPress 3.9 show up in your dashboards very soon.
Finally, as noted in the post, do check out Mike Schroder's Week in Core summaries he's been doing. They are an outstanding way to get caught up on what's going on. I hope these continue well beyond the 3.9 cycle.