What makes a good fantasy series?

I just finished reading the Beyonders series with Chasing the Prophecy, the third book.  I've also been reading the Harry Potter series to my son for the past few years, and we just started the final book, The Deathly Hallows.  So, I've been thinking a lot about what makes a good fantasy series.

I should point out that I mostly focus on YA and MG fantasy - I do read some grown-up (I don't want to say "adult") fantasy, but not as often.

I've posted elsewhere about the importance of a good "bad guy."  A fantasy series really needs a good villain, more so than most other story types that I can think of.  Voldemort is a good example - he kills anyone and everyone, and he's cruel to his own minions.  He has no friends and doesn't care.  He tortures the other orphans in the orphanage and enjoys it.  He even tries to bully Dumbledore, when Dumbledore shows up to find out if he should be invited to Hogwarts.  And what about Snape?  He really had no reason to kill him except a vague theory.  It was kind of like an experiment - maybe killing Snape would give him mastery of the elder wand, maybe not.

One of the reasons that I enjoy Harry Potter so much, I think, is because Voldemort is so thoroughly and convincingly bad.  Lord of the Rings is a really good fantasy series for a similar reason - Sauron is so overwhelmingly powerful.

Does it work the other way?  Sure - to take a really bad horror movie example, one of the reasons that the movie Squirm is so ridiculous is because evil worms (not even turned into giant monster worms!) are killing people in ridiculous "accidents."

I suppose there are no universal and permanent rules when it comes to something like this.  And I'm thinking about a couple of different series where the evil seems embedded in the "world."  Like Holly Black's Curse Workers series, where the main character is not exactly a nice guy.

There's a lot more to say here - but I need to re-focus and come at this again in a different way.