First, this a really pretty series. Not animation-wise, which is mostly just acceptable (with some awful CGI mixed in between), but the overall aesthetic honestly looks good enough to eat. The lighting, scenery, and colors, are all beautiful. I said in a tag of one of my posts that I had to restrain myself from screencapping the entire thing, and that's no exaggeration--there was some shots I saved just because they looked so good.
Action scenes were a mixed bag. Some of the excitement of them had the edge taken off due to animation budget constraints or (most of all) interrupting dialogue, others just kicked ass. I've heard the episodes with the giant, Lovecraftian monster Caster summons was met with some disdain from the fans, but it was there that I enjoyed the action the most. Nothing truly outstanding, just standard hack-and-slash, but sometimes hack-and-slash is all you need.
Where the series faltered for me was in the plot, and how much it overtook the dialogue and getting to know the characters. I didn't care about the Holy Grail War the moment I learned it was an all-out battle royale to achieve it, because from there on out I knew these were going to long-lasting characters, and thus knew from here on out it was going to be a lot of padding and complications over the war and rules behind it to meet its 24 episode mark. The journey should matter more than the destination, but the journey was so obsessed with its destination that I found more than a handful of scenes and sometimes full-episodes to be a chore to get through.
What made me decide to finally pick up the series wasn't all the Rider and Waver on my Tumblr dash (as fun as they were), but Kariya. Something about his sacrificing himself to a horrible fate to save a little girl and having to live with a decaying, rotting body intrigued me to no end, but sadly, the reality fell short of meeting up to expectations. Kariya's arc started out superbly and sounds fantastic on paper, but ultimately suffered from lack of information, screentime, and, most deadly of all, convoluted misunderstandings. I do think it's possible to have misunderstandings be a driving conflict without it being infuriating, but it really needs to earned, rather than sprout out of nowhere. And then there's the extremely uncomfortable entitlement/Nice Guy issues regarding Aoi (who also needed at least as much screentime as her husband) which weren't as laid on thick as I dreaded they'd be, but there's still that undercurrent of it that can't be ignored...
Despite all this, Kariya still ended up as my favorite, I think a lot for what he could have been rather than what the series gave us. I like the idea of a character whose tragedy is both a result of circumstance and of his own doing, who subjects himself to horrible things to save a person that ends up destroying him, who does things in the name of good but also isn't above doing self-serving things at the expense of others. I just wish we got more insight into his character and history.
The series itself wasn't entirely void of Kariya moments that got me attached to the character, however. I loved the scene with him telling Sakura that he's not as strong as her, bringing Rin back to her mother, the gradual decay of his mind that became more apparent as the series progressed, and his death actually really hit me where it hurt. The first time an anime made me hurt in a while, really. It's just a shame that his arc couldn't have been more fulfilling.
And that's about all I have to say about Fate/Zero. I liked Waver, Rider and Rin, Caster and Ryuunosuke were entertaining, Gilgamesh and Kayneth amused me,and the rest of the cast I'm just kind of indifferent about.