I don't have much to add, but I want to echo what PMSiobhanBR said and maybe pitch in an example from my own system.
Knowing what job each list is there to accomplish is key. Another reason you might have attention on the Someday/Maybe list is that there are things in there that aren't either a project or a next action, but fall in *some* other category. Maybe you need more lists for different purposes or jobs. Such as "Books I want to read" or simply "Thoughts" you don't want to lose but don't know where to put.
I'm a voracious reader, so I have a list separate from my main Someday/Maybe where I put all the books I want to read and even categorize them by area of focus, who recommended it to me and what they got out of it, and why it seemed relevant at the time. I keep that list in an OmniOutliner file.
Then I have a monthly routine where I go through that list and put together my study plan for the month, which then becomes a project (I read the books, take notes, derive projects or checklists and then share and discuss the books with my spouse and friends to really make the content stick).
Before I came up with this system, I had all books on my Someday/Maybe list and I just couldn't get S/M of my mind!