One element of the current publishing climate seems to be a tendency in genre fiction at least towards long series. It makes sense for the author, because they know that they'll have a readership for their characters. It makes sense for their publishers (If they have publishers. This is very much a trend in the independent market too. Perhaps even more so). It even makes some sense for readers, because they know what they're getting.
I've written series for other people, although for myself I've so far not gone beyond a single sequel. I like to be able to have a custom world for the story I'm writing. As a reader, there are also a couple of things that I watch out for in a series:
The first is for books that only make sense if you have read the rest of the series. Where everything in them relies on things before it, and where all the in jokes, internal logic and so forth are so established that the new reader can't jump in.
The second is the 'linking' book. That's the book in a series that only exists to get to the next one where the big stuff happens. It's a book long set up, where its own plot is secondary at best, and suggests that the series arc has taken over from the story arc.
The third one is the tendency to reset. In soap operas, characters don't grow quickly, if at all. They might 'learn a lesson' in one episode, but by the next, they're still basically back to being themselves. When characters gain more power but don't really change, that's a sign of the author wanting to preserve the central dynamic of a series.