I only like series that have a planned end if they're all about the same characters titles that tell you that there will probably be twenty more books, don't believe that this boyfriend is the heroine's true love.
I wouldn't mind this quite so much if so many series didn't feel the need for every book to have a "happy" ending, so the heroine gets together with her current squeeze/ex/guy who smiled at her in the line at the supermarket and all is well... until the next book starts at least. Unless the books have a habit of large time skips (which some series do well), it makes the heroine seem rather flighty.
Or hero. A lot of male protagonists suffer from this as well.
Tamara Myer wrote a funny series about a Mennonite woman who solved crimes at her B&B. Her sister had left the community but was around more now than when she actually belonged...and of course she wore mini skirts and lots of makeup. Our Heroine was in love with a member of her community. They even got married. Until, of course, she discovered that he was married to someone else (or cheating or something) and got divorced. Then she took up with the local doctor. And in between, there were always these very attractive men who kept showing up and...well, I didn't want her to stick with the unfaithful husband, but it would have been nice if there was someone with whom she COULD stick.
Count me as another one who is tired of Stephanie Plum's constant back and forth between Joe and Ranger. Also, it may just be me, but the later books seem to be thin rehashes of earlier plots--which were meatier and more serious (while still being funny).
Hamish MacBeth continues to carry the torch for Priscilla, but flirts with every female that comes thru
Loch Duhb (am I the only person who hears it as "Locked Up"?). After a while, instead of being suspenseful, it becomes annoying.