Lots of things make or break leagues in Detroit...I'll see if I can cover the important ones...
MOST IMPORTANT: Location
So ok, there's these two courses not a mile by road from eachother. Wagner park has a thriving doubles league on Thursday nights, Starr park is a ghost town. I've tried to get people to play a league there (I started it and tried to run it), because it's a better, more challenging course than Wagner. Who showed? Me and my ride. Our all-time high attendance is 4 people.
If people don't WANT to play a course, they're not going to pay to play league there even if they're going to be taking money home.
Also important is picking a course LOTS of skill levels can compete at. The best league in Detroit is at an 18 hole pitch and putt, and it's the playability by the lower divisions that's not only helped it stay alive and grow, but helped the AM3s and AM2s deveop more quickly and to a higher level than they might playing somewhere that has a lot of shots that are out of their range or too difficult for them.
Next to think about: Format and skill level
These kind of go hand in hand, and I'll explain why. There's a league in the Detroit area that's a variable format league (they play what they feel like that week). These are real casual guys - not casual in the sense that they're the type of casuals we all hate, but in that they just want a fun simple league that nobody takes seriously. Thing is, most of them want that because they can't compete in normal leagues - so when you're laying money down to make some cash, and it's alternate shot (or even worse, worst shot) doubles, and you're stuck with captain 200 footer, it just doesn't work. I've seen sponsored pros end up in 5th or 6th against 800-900 rated players just because they got stuck with someone who'd have a rating of 6 if they ever signed up for the pdga.
Nobody plays that league either, except for maybe 8-10 guys.
The best leagues are well run singles leagues, with tracked stats. That's what people are expecting when they think leagues, if they're not a random draw doubles league. The best league in Detroit is the Monday night Motor City Chain Gang league at Firefighter's Park. This year we've only seen less than 100 people there a small handful of times (due to weather). It's $20 yearly for membership but you get a shirt and a disc with league logos for that. If you're a member, it's $1 weekly to play, non-members pay $2. We've got a normal ace pool anyone can be in (2/3 payout) and a super pool that I think is members only that's 100% payout.
The only thing about running a singles league I'd suggest is if you have an open (cash) division, don't use a handicap for that division. For the AM divisions go ahead, but from what I've seen open players just bitch and moan if they get beat on handicap, even if it's only to win $20.
Also really important is having a bunch of people that are fun to play with, but that's something that you don't have control over, and that will sort itself out naturally anyways.