Series that start out promising, but a few books in you realize that either the author had no idea where to take it or is milking it for all it is worth. These are all the more painful because you keep going on to the next book in the series till you realize there will be no satisfying resolution, and that the initial connection that you felt when you started the series is long gone, and it is not just fatiguing and pointless to continue, you have also wasted many days and nights of your life, and you'll never get them back.

Then there are those other series that should have been culled at the first installment, and you have no idea why you ever finished the first book, and what streak of masochism induced you to pick up the next one. . . .

Ultimately, in either of these cases, you have to read some other book (or start some other series!) to get rid of the bad taste that the last one left in your mouth.

P.S. It will be appreciated if people add only the first book of the series (even if it is not the one where it started going bad), so we can have a clearer list.