I want to start off by saying, in comparison with a lot of other peoples opinions and tastes, I think I am really fairly easily pleased by books.
I know there are plenty of books(From both BL and elsewhere) that I absolutely love, and others hate.
(For example Lord of the Night dislikes the Sword of Truth series while I really enjoyed most of it.)
I honestly like books that have a good bit of bolter porn, and I also like books that are more complex and in depth.
Now, to answer your question, I think that good Horus Heresy books(and 40K books for that matter) are either made or broken by the characters.
I have only read the first 6 HH books, and I loved all of them except Descent of Angels, which I still enjoyed.
To me, the characters of Descent of Angels just weren't written as well as in the previous 5 novels. Now, that is just my opinion, and I am sure a lot disagree with it.
Also, I think that individual fans interest may have something to do with it. I personally cannot stand the Dark Angels and am only reading their HH books because they are HH books. Because I already have a dislike for the characters involved, I am a little more critical of it. I also love the traitor legions(Emperor's Children are my personal favourite) and I absolutely LOVED Fulgrim and bought a second hand copy of Aurelian for, honestly, way too much on ebay because of my love for the traitor legions.
Although, my favourite bits so far are WHY characters fall. I don't need grand literature that will be read for centuries to come. I just need a book that I can sit down and have fun reading.
Props for making this thread, I look forward to hearing some peoples opinions!