YA has a lot of series. Some say too many, but it's always been this way and I don't think it's likely to change soon. When I look over my wishlist of upcoming books, almost all are part of a series; so instead of debating series vs. standalones, I've been thinking about the different formats a series can take.
In all the series I read when I was a teen, the individual books were basically standalones. The info you needed to know about Sweet Valley High was given in a expository paragraph and then it was straight into a single story, which was wrapped up in one book and had little to do with what came before or after. It didn't matter if you read one SVH or a 100, or if you read them out of order - you got a complete story in one book. And the series could continue indefinitely, as the characters would always end up back where they started, ready for the next adventure.
I may be wrong, but I think this is the rarest kind of YA or Adult series today. This format may still be popular in Middle Grade (and some argue that the SVH books were MG but I don't agree with that. Remember all the attempted date rape, people!), but the only series I've encountered like this in recent years are the Stephanie Plum books and The Ladies No. 1 Detective Agency. If the books are good stories and well-written then I can enjoy this kind of series, but I find it difficult to get fully invested in them. Because it doesn't matter if I read every book, I tend not to. There's no urgency to find out what happens in the end (because there is no end), so it's more likely I'll let my commitment to the series slide and let years go by before I pick up the next book - if I pick it up at all.
The most popular series of the last few years have been the kind which do contain individual plots, but there is a long-running main plot that stretches across the series, and builds with each instalment. If a new reader were to pick up Harry Potter and the Half-Blood Prince, they could, perhaps, just enjoy a story where a boy wizard finds a mysterious book by the Prince and learns new spells from it, before discovering to his horror that the Prince is his enemy. But, for those of us who followed the series from book one, there are subplots to do with Voldemort and the preparation for the final battle we know is coming. And most importantly for me, every scene in the book has more emotional weight if you know the history between the characters.
Series like Harry Potter, Vampire Academy and Twilight appeal to me more because there is emotional continuity between books. A newbie could follow the plot of Shadow Kiss or New Moon as well, but you get more out of it by knowing the history of Rose/Dimitri and Bella/Edward. Series that follow this format are my favourites - I'm usually invested in the characters and want to see their story develop and know each book will give me a new development. However, as each book also tells a complete story, I feel satisfied after reading each one and it's my choice whether I want to finish my journey with the characters there or carry on. A great example of this is The Hunger Games: In the first book, you get the story of Katniss' first Games. There are threads you can choose to follow to the 2nd and 3rd books, but you can read only book one if you wish.
My least favourite type of series is the kind I seem to be encountering more and more in YA. In many reviews, I find myself reading/writing "Maybe this will be explained in the second book" or "We'll have to wait for the next book to find out what happens". The Demon Trappers and XVI were 2 series I started this year, where not much happened in the first instalment and it seems we'll have to wait for further instalments if we want to see the story take off. I understand some authors are telling stories that are too big for one book. And the continuity across books should make me happy. But if I don't get a complete experience in one novel (a beginning, a climax, a resolution), I can't help it - I feel cheated. Like I've been given a meal that consists only of starters, with no main and no dessert - I have to go back to the restaurant in a year's time if I want those. Some series that start out as Type 2, end up becoming a Type 3 later on. The Hunger Games can be read on its own, but there was no point to reading Catching Fire if you weren't going to read Mockingjay. I found Hush, Hush to be an enjoyable dark Paranormal Romance, but Crescendo made little sense and probably won't until I read Silence.
It's true that I'm likely to finish these series through to the end because I hate being left hanging. So in that sense, it's a win for the authors/publishers. But it also runs the risk that I'll stop enjoying the books and thinking of them fondly - if the series stretches on too long, it can become more like a burden, having to read so many books just to find out what happens.
So ideally, I'd like to read mostly Type 2s, with a few Type 1s and only very, very, rarely take on a Type 3. What about the rest of you? Do you agree that Harry Potter and Vampire Academy can work as standalones? Does that make them stronger or weaker for you? Do you prefer series that are episodic or can you not wait to dive into a long, complex story stretching over 7 books?