Although this series isn't perfect, I enjoyed it a great deal.
The Battle Sequences: Extremely well done. The final battle to shut the portal and Furion's first battle with Xavius in particular stood out.
Characters: Krasus and Rhonin were sort of annoying and sueish, but I didn't really hate them, because they were sort of likeable. Of course Krasus getting his ass handed to him a grand total of three times probably helped, as did the fact that the main characters (Furion, Illidan) do actions that equal (and in some cases exceed) anything Krasus and Rhonin do. Still, it left me with a bit of a taste in my mouth. His characterization of Illidan in book 1, as well as his slow fall into madness was well done, and Furion didn't annoy me seeing as 10,000 years pass and people can change greatly over one lifetime. Brox was interesting, although the fact that he got over his doubts quickly were a bit jarring. Deathwing, Azshara, and Jarod had great points of view, and they don't come across as flat. Although Dath'remar is still too damn nice in my opinion.
Writing style: Yes the prose is flowary and descriptive, but Knaak actually makes it work. The descriptions pull you in and convey the point, yet they never reach Paolini level.
Overall, it has weaknesses, but is ultimately an enjoyable series and a worthy addition to the World of Warcraft.