A strange novel who's "failing," if it can be called that, may just be a lack of real ambition. The novel is written from a "once-removed" or reconstructive perspective. This is nothing new and nothing inherently to complain about, many great novels have been written like this (example: Doctor Faustus by Thomas Mann, but that novel is written so because the "protagonist" is compromised and in this case taken by the devil, so a third party really brings us closer to the action). But here I'm not quite sure. And again, I'm not sure if this is a "failing," but it certainly leads one to feel a bit disappointed. As many people have pointed out, it'd be better to read Ka's poetry than read this novel, but of course what he have is this novel. I don't think it'd be fair to say that this is simply a failing of the author: "he wasn't good enough to write the poems so he wrote this novel from a distance." That seems too easy, and after all, is Pamuk really felt like he should have written the poems he would have, maybe they would have been good, maybe not.
But instead we have an outsider looking into a sea of passion (but not quite blind passion). The town is full of radical Islamists, radical republicans, radical Marxists, and radical lovers. But they all seem to understand the need for each other and the fact that everything is political. The Islamic Students realize that they need atheists to run the state, the political showman need the Islamists to be "moved" by their work, Ka realizes that Ipek needs him to get over Blue, etc. In this world Islam is political and atheism is a religious statement.
The problem and issue seems to be that we are all more interested in radicals (be they terrorists, lovers or actors or poets). While nominally this is a novel about extremes, on closer look it reveals itself not to be at all. It is a novel about the social and political interaction of extremes. This is inherently disappointing, we'd all much rather prefer to delve into the radicals and read Ka's all consuming poetry.