Close, but I think you are missing something.
BP is successful because it's got an intriguing story. Not only that, but it portrays it in a way that is fun. The missions are generally unique and varied so the campaign doesn't get old. No one gets sick of doing the same type of mission over and over and over again. It also hooks people with a healthy dose of "OMG, didn't know FS could do that!" too. It is always very well tested and generally solid to play. When you load up BP, you know you are going to play something that generally won't leave you with a broken, never-ending mission.
When the story fails, the missions need to draw the player back. If the missions get boring, then your next bet is some sort of "ooooh!" factor. Barring that, it should at least be well tested, but that's a long shot.
So, in short... Intriguing story, varied missions, ooooh factor, well tested.
--------
I would say that a big reason I never finished SG was because it was the same thing over and over. And, as you have said, it's a loooooooong campaign.
So what HLP really needs is people who can FRED with patience. It takes time to create multiple mission styles and to test.. that's not even to mention any sort of story (and the never-ending tweaking that happens with it). I feel like a lot of the single missions I played were guys who whipped up the mission, didn't thoroughly test it, and when I posted feedback about ways to make it more solid, they didn't want to take the time to put that to practice. It's work to make sure missions don't have dead spots or broken event logic.