As I see it, there are several different types of series, and each have their advantages and disadvantages. I’m not talking genres here—I’m talking about the actual format of the series. And, because I’m like that, I figure I’ll try to name each one (mostly just for the purpose of discussion here).
Type One: The Saga
I’m borrowing this title from Lee Modesitt, who talks about his main series (The Saga of Recluce) in these terms. A saga by this definition is a series where the books are connected only in that they deal with the same world. A saga jumps around in time quite a bit, and rarely follows the same viewpoint characters for more than a few books. The author is really telling the story of the world and its history, rather than talking about a specific set of characters. The most famous example in sf/f would probably be the Pernbooks. You rarely have the same main protagonist from one book to another, and the books were not released in chronological order. Recluce, mentioned above, is another excellent example of this kind of series. Discworld would fit here too, though those do have a larger number of consistent viewpoint characters than some of the others.
I tend to like sagas because of the way they mix a standalone with a series. You can pick up any Pern or Discworld book anywhere in the series, and generally you’ll be all right if you’ve skipped a few. Continuity between books is less important than the story of the contained novel. If you do read them straight through, you gain a little bit more understanding, and can enjoy the ‘series’ aspect of it that way. Of course, you always have the trouble of deciding whether to read the books chronologically as written, or as they happen in-world. (Personally, I vote for as written. Dragonflight isn’t the same if you’ve read the books about the arrival of mankind on the planet. Though, thinking of Pratchett, perhaps it would be interesting to start with the first one chronologically. What would that be? Small Gods?)
Type Two: Serialized Epic
These are the series where the next book in the series picks up right where the previous one left off. In essence, the author is writing one enormous book, releasing it in installments. Obviously, The Wheel of Time would fit here. Lord of the Rings is another great example. I’d probably cheat and slip Mistborn in here, though there are year gaps between the books. One essential feature of this type of book would be distinct, continuing characters and a continuing story that arcs over the course of several books.
The Serialized Epic is great because of the way it lets us invest in a series and characters over the long haul. The biggest problems are probably learning curve (you can’t just pick up the middle book in a series without being terribly confused) and cliffhangers. It’s extremely difficult to walk the line between giving each story its own arc and giving the entire series a larger arc, which leads to a lot of frustration as the books take time to wind down. However, despite its flaws, when this is done right it’s my favorite kind of series.
Type Three: The Continuing Adventures
This is the series where you get one central protagonist who has a complete story in each book. Then, when another book comes out, that character can go on another adventure. It differs from the saga in the fact that it goes chronologically and focuses on a single, central viewpoint character. The Dresden Files and the Miles Vorkosigan books are great examples of this type of series. Any given book could be the end, if it had to be, but we keep coming back to read more about the central character. Sometimes, there are longer arcs across books—but those are always secondary to the mystery or objective in each given volume.
These kinds of series can be very successful. (In fact, outside SF/F, I’d say this was the standard way of doing a series.) You gain even more flexibility for drawing in new readers with each book, as it doesn’t really matter where you start in the series. Sagas can get confusing because of their odd chronology, but the Continuing Adventures books are much more streamlined. However, they do usually trade some measure of scope and urgency to gain this format. I’ve found that I’m not as quick to run out and buy the next book in a Continuing Adventures book because the question of “What happens next” isn’t as strong as in the Serialized Epic. At the same time, I don’t get as frustrated with these series for not getting a volume out on time. They aren’t as confusing as the Saga, but at the same time, you don’t get to see the sheer scope of the world in the same way as you do with a series that jumps around through time and shows a lot of different viewpoints.
Well, I think that’s gone long enough. There are other types of series we could define. The Sequel Series (where an author follows one series with another) could be one, and Standalone novels are a different thing altogether, but I think this will do for now.