The previous version 1.6.0 had great new features, first and foremost the parallel up- and download of files and a way more performant handling of the local sync journal. That required a lot of code changes. Unfortunately that also brought in some bugs which are now fixed with the 1.6.1 release.
On the windows platforms, we experienced a memory leak that over time let the memory consumption of the client grow. Also, a problem in the Qt5 library that we ship for windows caused the problem that under some circumstances the wrong encryption lib was loaded, so that some people saw SSL problems on Windows.
And there were crashes. Users kept on reporting that the client was crashing after some time on windows, without a special reason. None of the developers was able to reproduce that or ever saw that. We asked for backtraces, which also can be produced on windows. Even though the backtraces looked similar, we did not find an obvious reason for the crashes. Finally, by reading through all involved code levels again and again, Olivier was able to spot some code in libneon that, again under special circumstances, could cause crashes on win.
Dragotin’s Blog: http://dragotin.wordpress.com/2014/06/30/owncloud-client-1-6-1/
ownCloud Changelog: http://owncloud.org/changelog/desktop/