There are two "seasons' worth of episodes though it is really one contiguous story rather than two sub parts. Fate/Zero feels like a much more mature and dark version of FSN. Most of the protagonists are much more mature, and quite dark in demeanor. Don't expect a lot of fights, in place of the weapon spamming you have intricate relationships and complicated plans; all ripe for betrayals and twists.
Fate/Zero is a very involved watch; there are so many side stories and plotting and counter-plotting, many of which take place during complicated dialogues. However, I felt it was a very worthwhile watch both in addressing some of the questions in FSN as well as standing in it's own right. Despite already knowing how the war would end, I was still immensely captivated by the intricacies of the masters' attempts to win the grail.
If you're looking for an involved and extremely well animated series with a slightly dark overtone, definitely check this one out.
Animation:
Ufotable has come out with some immensely impressive animations, Kara no Kyoukai being the strongest impression. They do not disappoint here. Fate/Zero is bathed in a gorgeous palette, highlighting the dark, gloomy outlook for many of the masters and the city as a whole. Everything from the fluid fight sequences to the superb usage of color effects and shadows is well done, and many levels above what I was in FSN. One of the most memorable scenes for me was at the very end of the first season where Velvet and Rider were having a conversation under a setting sun. If you missed it, I highly recommend re-watching for it as it is one of the most convincing and fluid sunsets I have seen in an anime.
As far as characters go, there are a lot of older men in this series than FSN. I guess they sort of balance it out by having loli Rin Tousaka and an even smaller Ilia. In any case, one of the early challenges in this series for the viewer would be to figure out how to discern Emiya Kiritsugu from Kotomine Kirie- as both have deep voices and the look of death on their face. As far as consistency goes, Saber and Archer keep their appearance from FSN (though Saber dons a bodyguard suit rather than her casual attire). Lancer is a harder one as his outfit is in my opinion far superior to the one he dons in FSN.
This series has a lot of blood and a significant amount of random slaughter, though the camera does pull away at the most grueling moments. This is much too dark of a series to have significant ecchi.
Sound:
Despite the huge upgrade in animation quality, I don't believe the music in FZ was as catchy as that in FSN, though the quality in general was still very good. There is an OP and ED per season. All the pieces are fitting for the series tone but I didn't really like either of the OP melodies (sorry Kalafina). The EDs on the other hand were both good, especially the second series ED by newcomer Lunamaria er... Luna Haruna, composed by Yuki Kajiura.
The voice acting is superb, my only complaint being that Kirie and Emiya sound too alike (though it may very well be on purpose). The VAs for the heroic spirits from the 4th Grail War pick up their corresponding roles here- and Kana Ueda even returns to voice chibi Rin! All the voices were appropriate given their character personalities, though I do believe Lancer turned out to be much more of a "heroic" figure in FZ.
Story:
Fate/Zero is a pretty focused series, set aside the two-episode adventure through Kiritsugu's past and loli Rin's adventure. While the obvious plot line is the battle for the holy grail, the series spends a lot of time following each individual master as they plot and plan their next move. It really makes you wonder how Shirou lasted so long in FSN... In any case, the entire story is pretty much seeing how each master's plan weaves with each other- this is the fun, figuring out which plan will win the day or how someone's plan will be ruined.
In terms of tonality, F/Z is much more in line with that of Kara no Kyoukai than FSN. Gone are the humor relief moments and teenage crushes, replaced by a single-minded purpose of winning the war. Likewise, in terms of scale, the masters and servants of F/Z seem far superior in ability and talent than their counterparts in FSN.
There are numerous plot twists, many of which are masterfully executed, putting lots of other recent series to shame. The series does (eventually) build to a final climax, bringing the clock back to "zero" and thus setting the stage for what happens in FSN. It is a very well rounded storyline though a few answers do go unanswered near the end.
Character:
If you've watched FSN, be ready for a lot of early surprises, character-relation wise. Fate/Zero focuses quite heavily on some of the character relationships and does a great job fleshing out the backgrounds of some of the masters and servants, while leaving others shrouded in a veil of mystery. While giving some more background on some of the supporting characters would have made their eventual demise more impact, perhaps they felt the series was long enough already. I was particularly keen on the large amount of time spent on the Velvet/Rider relationship, especially considering their relative unimportance for FSN.
There is an interesting tension between many of the servants and masters, as their fundamental beliefs in how one should behave clash at multiple points. It is very interesting to see how these mismatches affects each pair's fate. However, most interesting to me was how the characters related to one another, and especially how those ties related to the masters in FSN.
The actual main characters are all very strong. Each master and servant have very memorable and distinct personalities. In particular, it was interesting to see Kiritsugu's dark notion of justice, ironically passed on to Shirou in a much more "hopeful" spirit.
Value:
Fate Zero is hands down the series to watch in its season/s. It combines great tonality and animation with superb directing to create a piece that you'll definitely remember.
