For me, it's got to be Raising Steam, by Terry Pratchett. So sad to see the severe deterioration in a great author's work - IMHO he should have stopped at Making Money, as the last three Discworld books are definitely inferior to the glory days.
This year, when I went through quite a few books bought years ago, I didn't have the courage or the patience to carry on with:
-The Chrystal Singers by A. McCaffrey (The heroin and the style annoyed me no end. I left 100 pages before the end.)
I was completely disappointed by Allegiant-Veronioca Roth.
I absolutely loved Divergent, Insurgent was okay, but to switch writing styles AND *spoiler alert*?kill off your main character at the end
Ugh. Was not a fan.
The Forge of Darkness - Steven Erikson was most disappointing because I had such high hopes for it. Lots of back-story delivered but without the Marines or similar characters to add some humor it was often hard, slow going.
Worst - Fifth Business by Robertson Davies - I should know better than to tangle with "literature"
Started to read "The Hunger Games", thought it poorly written and a big let down.
Give up at less then 100pages, felt the same about "The Curious Case Of The Dog In The Night".
Interesting how mileage varies. This book, and the Deptford Trilogy are some of my favourites, and have had multiple re-reads over the years.
Big fan of Robertson Davies. I always found his work very accessible.
I've never read the Hunger Games (although I watched the film on a flight and found it surprisingly good), however, I felt that the Curious Incident was one of the most unputdownable books I have ever read. The description of Asperger's/Autism was fantastic, and the little bits and pieces throughout were really clever. I saw the play last year and thought it was even better.
I think my biggest disappointment of last year was Flood by Stephen Baxter (not written in 2013 I know). I read it after The Long Earth, which I very much enjoyed. I had no empathy with the characters in Flood, and found I didn't really care what happened. Brilliant concept and ideas, but disappointing execution.