Betrayal

The New Jedi Order series, for its faults, did a lot to advance the Star Wars universe by shifting the focus from the characters in the movie to the next generation of Jedi, and by introducing some interesting moral relativism that forced the characters to think beyond the good/evil dichotomy of Jedi and Sith. Betrayal seems to undo a lot of that - the kids are still there, but the focus seems to be primarily on Luke, Han, and Wedge.

Jacen's Sith turn also seemed sloppy and rushed - although Lumiya presented it as a continuation of his studies in moral relativism, it clearly isn't, because it requires him murdering a former apprentice in cold blood. I may be biased, because Jacen has become one of my favourite characters in the entire saga, but he just seems out of character here.