You mean besides our catchy song and cool colours right?
I think we've generally had a large fan base but just struggled to convert those into members - understandable when you've had 3 decades of SFA. Back in 2010, we only had about 35k members so we've effectively doubled our numbers since then and I think a large portion of that is due to the 'modernising' of the club. With sound operators like Benny Gale (CEO), Gary March and then Peggy O'Neill we've been kicking a lot of goals off field (clearing debt, growing our membership numbers and revenue streams).
I think the important ingredient is it seems most of the key people at the club are in sync with each other and the direction of the club should be heading. Think that is also supported by our relatively affordable membership prices and a decent (by Richmond standards) footy department/coaching set up - 3 (if not shortlived) finals campaigns is very good by our standards lol. I think another underrated thing was having a clear vision and strategy. While some mocked it, I think the 3-0-75 plan was a big success (even if we fell short on 2 of the goals) as it helped drive the club to improve with ambitious targets to work towards.
I think the remarkable thing is if we can get that elusive win in a final, it might increase again.
I could be wrong about Carlton but from what I've heard from mates and co workers of the Blues affinity, it seems (at least when Malthouse was at the club) the supporter base is a bit disillusioned with some of the senior people at the club and the decisions they have made. I know a few who are still upset that Ratten was sacked, others who are dumbfounded as to why players like Betts, Waite, Garlett, Jacobs etc were let go. I think Carlton the club needs to (if they haven't already) communicate to the fanbase what they want to achieve in the next 2-3 years and try to get the supporters (particularly any disillusioned ones) back on side. Hopefully a new coach and a slew of early draft picks will help get some more fans back on the membership train.
I reckon if the club had a bit of a reboot at the top (maybe a new CEO, president etc to go along with Brendon), it might help in the long run. New ideas and fresh perspectives to help revitalise the club. No doubt if Carlton get their act together, there is no reason they can't get to 70k plus members also IMO.