Looks like the broad bronze category is intended to keep new players isolated from longtime players for quite some time.
In the older 'star rating' system, three stars was 85 quality or higher. Now it looks like it's category based, but there's still a quality rating. I still can't join matches less than 85 quality, so I hypothesize that the quality system was changed to be rank-based (i.e. you can't join a match whose average is more than five ranks below yours, but you can always join a 1650 or higher/diamond match).
Anyone know how this is designed for a fact?