The reason that, at least for me, the Zahn novels are so superior to the novels that followed is that Zahn actually wrote them as novels. Other authors seemed to think they were writing movies. Even Zahn has not been able to escape the taint of the prequels, and his recent stuff is medicore at best. Plus, as others mentioned, he fell in love with his own characters (Jade, Thrawn, Karrde)
I actually thought the Black Fleet Crisis was good, although it lacked the feel of Star Wars almost completely. Weird.
I loved KOTOR 2. Maybe my second favourite Star Wars experience after the original film! It built upon the more serious tone of the films that started in Empire and explored the Jedi and Sith philosophies in a mature manner. Great characters as well. Buggy as hell though, and essentially unfinished (the last 10% of the game is a complete mess).