The prologue is something that helps all three routes but hasn't really been given much attention in animation until now. Most of the prologue consists of Rin's internal monologues, which works fine for a novel but needed to be cut down for an actual animated project - and ufotable delivered, by replacing those internal monologues with directorial choices, usually in terms of body or facial language. They managed to squeeze out all the exposition that they needed to, as well as the foreshadowing that the prologue had in it. My personal favorite little touches were the BGM playing when Archer first appears and having him mouth "a certain phrase" that the audience isn't privy to (but we all probably know what it was) against Lancer.
My only real complaint is a nitpick in that I think the action is too fast. I get the idea - that it should feel like Servants are a step above humans, but when the action is so fast it can get difficult to follow and enjoy. Slowing it down a touch wouldn't hurt. Oh, and this is completely minor but for some reason Issei sounded a bit off, a little too deep. I don't know if it's just because Madono is getting older or if he was sick or something but he didn't really sound like the Issei I remember. Again, minor nitpick.
As for the music. I wasn't familiar with Fukasawa's work so I was a bit worried that it wouldn't be very good. I shouldn't have been worried. So far, I like the BGM better than any of the other Fate anime OSTs for the simple reason that it remembers the visual novel's soundtrack, and to me that just feels right. Many scenes eschew music entirely, which isn't uncommon in any media but it was actually noticeable here. Cutting the music was common in the novel, so having moments like that help it feel more familiar. Though I didn't catch much inside the calmer tracks, I noticed that many of the tracks seem to do well in reminding me of the visual novel. The composition when Lancer and Archer fight is the most notable one to me - if you listen closely you can actually hear elements of the original novel's "Gekitotsu Suru Tamashii" in the strings. Also, the composer has noticeably lifted a few of the more memorable musical motifs from Zero.
All in all, I am very optimistic about UBW. I really loved Zero when it was airing, but in retrospect I didn't enjoy it as much as I thought I did, but this is UBW. I know that if it stays faithful to the novel, it's going to be golden.