What makes a good series?

I'm a HUGE fan of the Fever series by Karen Marie Moning and she’s released a new book, Iced, that’s based in the same world. The main character in Iced was a secondary character that I didn't care for in the Fever series. As I try to decide whether I’m going to read Iced, I've been thinking a lot about what it was that made me love the Fever series so much.

If you haven’t read the Fever series it’s about an innocent young woman whose sister is murdered in Dublin under mysterious circumstances. Mac travels to Ireland to find out what happened and stumbles into a big freaking mess. Dark Fae want to invade the human world and Mac's ability to find a powerful book is the only thing that might save us all. So big stakes, right? That’s one thing. But in addition to that, there are several conflicts consistent throughout the series that keep you reading:

Power Imbalance

You're a sheep, Ms. Lane. Lamb to the slaughter in a city of wolves. - Barrons

The heroine, Mac, is hugely outmatched by the hero, Barrons. I should say here that he's not a hero in the traditional sense. This is an Urban Fantasy series, not romance. But the romantic subplot is engaging and becomes increasingly important as the series progresses.

Mysterious and morally ambiguous (that might be generous) character

I will chain you up, tie you down, leash you with magic, whatever I have to do, but you will help me get that book. And when I've got it, I might let you live.- Barrons

Or this

I'm not the hero, Mac. Never have been. Never will be. Let us be perfectly clear: I'm not the antihero, either, so quit waiting to discover my hidden potential. There's nothing to redeem me.- Barrons

While there are no unblemished good or irredeemably bad characters in the series, Barrons is the most shadowy of them all. And he remains that way. I was very happy with Shadowfever but I'm torn about Barrons. I both wanted him to be more fully revealed AND loved his mystery at the same time.

Sexual Tension – with believable obstacles to fulfillment 

“Was he a good kisser, Ms. Lane?” Barrons asked, watching me carefully.
I wiped my mouth with the back of my hand at the memory. “It was like being owned.”
"Some women like that.”
"Not me.”
"Perhaps it depends on the man doing the owning.”
"I doubt it. I couldn’t breathe with him kissing me.”
"One day you may kiss a man you can’t breathe without, and find breath is of little consequence.”
"Right, and one day my prince might come.”
"I doubt he’ll be a prince, Ms. Lane. Men rarely are.” - Barrons

The sexual tension kept building throughout the series. Even once they did have sex, his motivation was murky and she...wasn't herself at the time. So...more tension.

Interesting word building – revealed in bits and pieces

There's some great worldbuilding in this series. Big, complex worldbuilding. I'm not going to go into it now but there's definitely enough going on outside of all of the above to keep your mind turning.

So aside from creating a need in the reader to see the main characters triumph in what is plot-wise a pretty basic story (they're looking for a book), you also have the need to see the scales balanced between the hero and heroine (or at least evened up a little), the need to know who and what the hell Barrons is and the need to see all that sexual tension explode on the page. And NONE of those conflicts were truly resolved until the final book.

Also, all of the books ended on cliffhangers. Some people hated that, but it worked to ramp up the tension.

While I loved the Fever series, I'm conflicted about Iced. Dani wasn’t my favorite character and I cringe a little bit thinking about reading a book from her POV. I’ve also considered waiting until a few more books are released in the series before I dive in. But…my finger keeps hovering over the buy button.

Has anyone read it? What did you think?
If you haven’t, what is it that keeps you reading your favorite series?