Series writing is fun to do. It has a lot of advantages. Your main characters are already developed and you're comfortable with them. You know how they're going to react to a given situation. You know their quirks, their strengths, their values and their aspirations. No problem! All you have to do is figure out a plot that will showcase our intrepid main character. Even that's easier with one you already know.
The down side is that you already know your characters. You may lose some of your freshness, the reader may begin to find him stale. And you begin to run out of new ideas for him, so you find yourself repeating some of his lines. You sort of zone out while you're writing the book and wake up to find you're on the third book and they all sound the same.
So now I'm supposed to come up with some really nifty ideas for how to overcome all that. You can't present a problem if you don't have some ideas of how to solve it. First of all, remember some readers like that very aspect of a repeating character. Remember Sherlock Holmes? And now he's going to be on a new television series this fall. Can hardly wait. Sherlock Holmes was made complex enough as a character that we never grow tired of him. I remember I was shocked when I found out he dabbled in drugs, but it never seemed to diminish his capacity to solve murder mysteries.
How about Miss Marple or Hercule Poirot, and Lucas Davenport from the Prey series or Alex Cross. Repeating characters that make you wait impatiently for the next book starring them. I find that as I read these characters over the years, they change. My perceptions of them change. They mature, get married, feel lonely, question themselves, try and sometimes fail, although in the long run, they prevail, make mistakes along the way and change their philosophy on things in general, just as we all do as we mature. I like a character that grows with me. With repeat characters, you might keep that in mind.
Secondly, you must spend more time on your new characters for the next and the next books, the murderers, plotters, evil doers introduce new characters that are compelling enough to draw us in as well as repeat character. The interactions of your repeat character and the new villian bring new excitement to the story, yet because you know your hero's strength. He's solved these problems before, you have no doubt he will again.
So don't be afraid of a series. The reading world can always use another Alex Cross, Hercule Poirot, and Miss Marple.