Not sure where the calibre comment comes from. Have you looked at when was the last time Dave (who has continued to work on Sigil almost continuously since the last beta) found the time to push a Kindle Collections plugin update?
I would happily post reams of information about the various changes, fixes and new stuff intended for 0.6, at least about the changes I have contributed anyways.
However as has been mentioned on another thread much of it is subject to further review before the official beta begins. Nearly every day we are collaborating and refining things, sometimes new features are being combined together as we get more time to think them through or attack other items on the list. We may yet find things need to change further, perhaps some may be pulled in favour of a longer term alternative. So a post we made now about feature xyz may confuse people further if it doesn't make the final cut or is called something completely different. Just look at the confusion caused by having the manual ahead of the official release last time.
As DiapDealer says there are no secrets when it comes to what is actually happening with the source code on a daily basis via the Sigil website. A starting point would be to just look at the Changelog.txt in the root of the source - you can see nearly 100 fixes, changes or new features listed in there currently for 0.6. Some of those "one-liners" were an awful lot of work too...