While it might not seem like much has changed, lots of behind-the-scenes refactoring has been done to fake future development smoother. Most notably, this refactoring allowed for the creation of a suite of unit tests, which should hopefully catch bug regressions before they make their way into new releases.
In all previous versions of JustTag, the preview window shared the same code for displaying images and videos, and this was a major source of complexity. Now the two have been separated, so images and videos are treated separately. This means you will no longer see a "play" button on images, nor will you be able to pan and zoom videos.
Finally, JustTag can now handle files whose paths exceed the 260 character limit. This was a major flaw in JustTag, since every tag you add to a file will increase its path length. If your file is nested deep in the filesystem(or if it is located inside a folder that itself has multiple tags), then this became very problematic. This fix was the biggest motivator for the undertaking, and I'm relieved that it's finally done.