Series? Seriously?

I'm running late again, today... but I'm here! And, I have a question. You don't have to answer, but it's something that I wonder about, as a writer. I know the answer for myself as a reader, but as a writer I'd love to know: How do you feel about book series (serieses?)?

Bear with me here. I know as a reader, I love to hang out with characters from previous books. I love to go back to the worlds that sucked me in the first, second or third times I visit the different worlds I read about. But that's me!

As an author, I love when a secondary character jumps up and down and hollers, "Look at me, look at me... I'm just dying for my own story!" I always want to go back to my worlds and revisit characters that spoke to me... As I"ve mentioned before, Megan and I have a series of books we're working on, and there are already a few characters' stories that we've mapped out, and we're excited to see them take shape. I think the problem we're running into (or maybe it's just me... lol, I am a bit obsessive) is that there are so many ideas we come up with, ideas that would be great to incorporate that it can be a bit overwhelming. What do we include? What would just be... minutiae? How much can be woven into a series of stories to keep you coming back, and how much "cliffhanger" would be too much to make you stop reading? Is it okay to have the immediate story resolved with the main characters of THAT book but still have some... mystery to keep you coming back for more in the next book? Help me out here... what would you love to see in a series?