Don't get me wrong. Rating novels and sharing opinions is perfectly fair and legitimate. When it comes to meeting the expectation of the OP, though - which was to have a way for an unfamiliar reader to know which book is "good" or "bad" - it's gonna be very difficult to arrive at anything resembling a consensus where most titles are concerned.
Lord of the Night, for instance, gave the "Gaunts Ghosts" series (comprised of 14 novels) a blanket "2" - the same grade that he gave "The Purging of Kadillus". If you were to ask me, though, "Gaunt's Ghosts" is one third of 40k's defining tripod of fiction (the other two being the Horus Heresy and the Eisenhorn trilogy). By contrast, I think that "The Purging of Kadillus" is lackluster without being bad, and a poor depiction of the two factions involved without necessarily being malicious or irresponsible about it.
"Prospero Burns", to mention another example, is a very polarizing novel. On this forum along, I've seen posters praise it as great fiction and denounce it as false advertising and a poor re-imagining of the Space Wolves in more or less equal numbers.
Honestly, I think a better solution might be for the Mods to open up a single thread with links to secondary threads - each devoted to a Black Library novel. Each novel's thread would have a poll whose choices corresponded to a "grade" (1 to 4, 1 to 5, 1 to 10, or whatever). Posters could thus assign that specific novel a grade and then offer their opinion. A curious would-be reader would be able to quickly get an "average" grade from the poll, and could scroll down to get stated opinions. The forum itself would only need a single stickied thread, which you would update with new links for new novels.
Thoughts? I realize that posters such as Lord of the Night and Bane of Kings - both of whom devote a lot of time to providing the fora with reviews - might be more qualified to speak on this.