My problem with Fate/Zero: the characters
One of the reasons that I wound up watching Fate/Zero only sporadically and without any particularly burning enthusiasm is that I found basically all of the characters to be, in Author's phrasing, jerkfaces. Out of the entire collection of Masters and Servants and secondary characters, the only one I actually found likable was Waver (and even then he's only truly likable after he's matured). Even the much-admired Iskandar is not all that nice when you get down to it, for all that he serves as a good father figure for Waver.
(The closest any Servant comes to being likable are Saber and Lancer but they're both what I'll call 'Heroic Stupid', each blindly heroic in their own different ways. They're both capable of doing cool things but that doesn't mean that I find them sympathetic.)
There are two areas of special failure that I want to single out. The first is that Fate/Zero reduces a number of characters to cartoon villainy or close to it, most noticeably Ryuunosuke and Caster but also people like Kayneth to a lesser extent. This is lazy storytelling and pretty much made these characters boring, which was not helped by the story's ham-handed attempts to make them vaguely sympathetic (often at literally the last moment).
(It's possible to make totally evil characters still be interesting, but it takes being clever instead of just kicking dogs. Caster pretty much just kicked dogs, metaphorically speaking.)
The second is Kiritsugu. The story's attempt to make him sympathetic by giving him a tortured background simply persuaded me that he had been damaged from the start. I felt that his actions and reactions were too unrealistic for anything approaching a normal child and the story didn't convince me that he'd been broken by excessive stress; instead I wound up feeling that Kiritsugu was a natural killer who had latched on to the idea of 'justice' as a substitute for any innate sense of morality.
(In the Fate-verse, this struck me as completely unsurprising for a mage and the child of a mage. Let's face it, a lot of mages in the Fate series are badly morally damaged; just look at Tokiomi's actions with Sakura for one example.)
(I was prodded to write this entry by Schneider's entry (via Author). I entirely agree with him on the Fate/Stay Night characters versus the FZ ones; the cast in F/SN is much more likable and interesting to me and as a result I found F/SN much more engaging than I did F/Z.)
Sidebar: On Iskandar
My view of Iskandar is kind of tangled. On the one hand, he spends a lot of time in the show being a likable guy and a good person for Waver to be around. On the other hand, I can't watch those segments without remembering what Iskandar ultimately believes in and that at the core he is not a good person (no matter how nicely he may act); instead, he is the King of Conquerors, larger than life in vices as well as virtues. I can't listen to even his early enthusiasm for taking over this new world he finds himself in without thinking about what his words imply.
(We may laugh at his ambition, but Iskandar is serious. He would throw the world into fire and sword simply because he wants it. I can't forget this even when he's personally nice to people.)