First things first, if you walk into this show expecting a masterpiece similar to Fate/Zero, you'll need to reevaluate your expectations. This show is by no means a bad show; in fact, it's probably better than most of the anime out there. But as a Zero sequel, it falls flat. This is the inevitable problem of adapting a VN, since an anime on a single route leaves out so much and never feels quite complete. Ufotable did the best it could with what it was given, but the show couldn't meet the standard Zero set.
What went wrong? Naturally, when adapting a VN to anime, you'll need to cut out ALOT of details. The visual novel was essentially alot of cooking, eroge scenes and story, and I'm glad Ufotable got rid of the former two. They could have, however, focused on fleshing out the major points of the story well, as this is where the problem starts. Shirou, the protagonist of the series, is never given any depth or good backstory; we simply get a few PTSD flashbacks that show us he had a hard time in his past, but we are never given insight into his mind. So, whenever he does something stupid (which is 100% of the time), you can't help but hate him and ask why he's so irrational. And since he's the focus of the show, he just brings the story down. Besides him, the other characters are actually interesting, and it's a shame they weren't fleshed out at the expense of Shirou.
The redeeming qualities of this show are definitely Archer and Gilgamesh. Archer combats Shirou's stupidity with actual rational thought and complex backstory, and Gilgamesh is a good villain like always. Unfortunately, the series does not provide any closure for alot of the things left open-ended in Fate/Zero (Kirei reviving the Grail and settling his dispute with the Emiyas/Matous, Saber redeeming herself, etc).
