It is unclear what the bug is, or how critical it may be, but it obviously must be relatively major for Apple to delay the release of the update. An Apple spokesman made it sound as though Apple knew about the bug and had hoped to be able to have it fixed in time.
We don't know when "shortly" will be, but traditionally the word has meant a few days at most when used by Apple.
The big additions in watchOS 2 are native third-party apps, third-party apps being able to display info on the watch face, photo backgrounds, time travel (aka looking ahead on info from calendar, weather, etc), nightstand mode, and more. So, users will have to wait a bit longer for all of that.