After a while, development gets feature-locked, meaning no more major changes to the OS. This is the stage that focuses on bug fixing and overall stability. If this was not done, then a stable release would never emerge, as new features would continually introduce new bugs.
10.2.0 got feature-locked around build 15xx, which then split into 10.2.1 for the team still working on new features. Think of 10.2.0 as a nearly-finished product getting polished off, and 10.2.1 as an experimental testing branch for new features to be added (and some removed if they don't work out). A few months ago, 10.1 was getting polished while 10.2.0 was the experimental build bringing us toast notifications and lock screen previews.
10.2.1 is currently working on some camera improvements, a new media file system, OpenVPN, and other features. Some are incomplete and do not work, much like the Screen Reader feature in early 10.2.0 builds. If you want something fairly stable, stick to 10.2.0.1767 which is rumoured to have been released to carriers. If you want bleeding-edge features and don't mine troubleshooting and the odd crash, then grab a 10.2.1 build and have fun.
10.2.1 is too unfinished at the moment to be a daily driver for everyone.
.1767 was the worst os I've installed , it didn't work at all for me. Wouldn't hook up to link or my computer for that matter. Always failed to load drivers. And it removed all photos I had. First time I had a major issue with a leak. I like the latest leak ,don't find it lacking personally. Thanks to you and the team for all your efforts!