Retention of scouts by having meetings only 2 times a month and doing most of the achievements in the den meetings.  Some of the fast start den meeting plans have you working on multiple achievements all in the same den meeting, so unless the den meeting is extremely long, it's gonna mean short cuts. Very disturbing trend In my opinion.

As a den leader, I planned meetings on my own for the first 3 years, then used the Fast Tracks programs for the last 2 years.  We have always had 2 den meetings per month, but you have to remember that there is also a pack meeting, usually a pack activity, and possibly an additional den outing/hike/campout/etc.  With an average of 4 or 5 scouting events per month, adding additional den meetings would have a negative effect in my unit.  The new program incorporates more activities into the den meetings, but there are still requirements that need to be done at home.  The difference is that the program reduces the amount of work that gets done (or doesn't get done) on their own at home.  I modified the Fast Tracks program to meet my needs, but it was great to have the foundation of material laid out for me.