The sad thing is that I'd been looking forward to this. I don't like the Nanoha franchise in general, but I'd been hoping that I might get more from a longer, slower, less action-oriented Season 3 that was less popular with the fanboys.
Nope. The fanboys were right. It's bloody awful... no, I take that back. That's not fair. It's superficially functional TV in places if you ignore the fact that Masaki Tsuzuki doesn't understand character-based drama. I'm reminded me of Star Trek, actually. Not the good Shatner stuff, mind you. I'm talking about the empty 1980s TNG stuff that eats away your soul as you watch it and makes you wonder what kind of world we live in that this could be considered broadcastable.
Let's recap. In Season 1, Nanoha was a magical girl! She lived in Japan with her friends and family, whom she loved very much when she wasn't stonewalling them out of her life and refusing to share possibly life-endangering information. At this point she's nine years old.
Six months passed between that and Season 2 (A's). Nanoha's now a bit older, her best friend is the ex-villain who'd previously been trying to kill her and she's paying ever less attention to her former friends and family.
Now, though, ten years have passed. Nanoha doesn't live on the planet Earth any more and there's no evidence that her family even knows she's alive. She works for the Lost Property Riot Force 6 of the Time-Space Administration Bureau, with the twin roles of "friendly but terrifying human weapon" and "recruit trainer". We join the story as four new recuits (Subaru, Teana, Erio and Caro) submit themselves for the Mage Test, which if they pass will let them undergo Nanoha's combat training. In other words, our heroes are officers and their subordinates in a not-quite-military federation in an SF universe. It's all terribly civilised and utopian... but our heroes are clearly part of a military chain of command. When things go bad, everyone follows orders and operates as a well-drilled unit.
You can see why I'm comparing it with Star Trek. (He says, avoiding the debate about whether Starfleet's a military or civilian organisation.) More importantly, though, it's like 1980s Trek in its avoidance of drama, by which I mean "stories or scenes built on characters acting on their desires, motivations or beliefs". I'll summarise the plot.
1. Trainees get trained by Nanoha and go on missions. There are action scenes.
2. Baddies do something bad in ep.17.
3. Our Heroes Strike Back.
...and that's it. Really. That's all 26 episodes. I started watching the show and found myself getting ever more perplexed. Nothing happened. Then nothing happened again. Then, in the next episode, the same. There was lots of dialogue and training fights and stuff, but none of it said anything about the characters. You could have replaced everyone with a Cyberman without changing the plot. This got so glaring that in ep.6 I decided to test my theory by consciously looking out for any character-based plot beat, no matter how trivial. I had to be wrong, surely? The show wasn't as empty as it looked, right? (This was on the spur of the moment, by the way, and my choice of ep.6 was random.)
In fact, there was a moment. After all that combat training, civilised conversation, backstory, etc. there's a tiny character beat near the end about paying the bill at a restaurant. (It's a character-based moment. It's not big, but drama doesn't have to be.) It lasted a few seconds.
So not entirely empty, then. It's pretty close, though.
It was an interesting exercise, though, since ep.6 is far from the worst in the show. It has a warm moment of insight into Fate. It's thoroughly pleasant and urbane. It convinces you that the Time-Space Administration Bureau is civilised and full of good people, which is very Star Trek. The episode is also an example of how Masaki Tsuzuki uses backstory, as his way of exploring character in the absence of, say, having his cast do dramatically meaningful things. This can also be seen in the mega-battles of the season's final run of episodes, in which the show tends to find its emotional power from the following formula:
(a) a fight involves a character with an emotional backstory
(b) character remembers backstory
(c) character wins fight
This is a pretty mechanical approach to writing, I think. However it works. The most effective characters (often ex-villains) all have some tragedy in their past, which lets us feel for them as they put their lives on the line. Fate can be almost heartbreaking with just a single flashback to her late mother, for instance. It doesn't always work, mind you, e.g. that ex-sniper's unintentionally funny flashback in ep.17.
What's particularly interesting (i.e. funny) is how Nanoha fits into this. I'm imagining a Nanoha equivalent of Fate's flashback sequence about her family. (Nanoha: "I have a family? Oh yeah, that's right. I forgot. Back on planet Mud or whatever. They're called... no, hang on, it'll come to me... damn, lost it. Well, it's only a few names.") You see, Nanoha doesn't fit the show's template. She doesn't have a tragic backstory. She was a normal nine-year-old girl with a warm, happy family background and you can't shoehorn anything heartrending into that without raising the awkward question of why Nanoha has never given a visible damn about anyone from her past.
Ep.9 invents a past trauma for her anyway, mind you. We've never seen it before. It involves a near-death experience. However the show forgets about it immediately afterwards and never refers to it again. What it does instead is to finds a new story role for Nanoha, which is both welcome and surprisingly effective. Firstly, it gives her someone new to care about. She finds and adopts a little girl called Vivio. (Nanoha has always been enormously nice and compassionate, after all. My merry character assassination doesn't match her apparent personality and only comes from bad writing in the earlier seasons.) Secondly, the writing starts showing slight signs of awareness of what Nanoha is. People are genuinely afraid of her, she's reluctant to adopt Vivio because (in a fascinating speech in ep.22) "I'm always busy with myself and I don't have the right to be a gentle mother" and she gets villains saying things like "is she really human?" and "are you a demon?" It's also noticeable that when Nanoha adopts Vivio, as far as Fate's concerned this immediately means that Vivio has two mothers.  However when Fate became the guardian of Erio and Caro, Nanoha made no attempt to be seen by them in a similar way.
 - Yes, Nanoha and Fate are an item. The show itself has maintained deniability so far, but Nanoha's voice actress and the show's creator and writer Masaki Tsuzuki have both said so publicly.
Anyway, the third cool thing this season does with Nanoha is to make her terrifying. No matter how badly written, she's always been the poster girl for destructive potential. Here she's the bloody Terminator. I felt sorry for one villain who got in her way. She's usually her usual sweet-natured self (and wearing a limiter in order to be allowed to do her job) but when she's not... get out of her way, fast. Note what she ends up doing to Vivio, for instance. "This might hurt a bit."
Don't just take my word for it, though. Read other people's reviews. People hate this series. Others love it, admittedly, because as always with this franchise it's got no-holds-barred fight scenes with Nanoha and Fate being awesome. Nanoha is, surely, the most badass magical girl in anime. The show's got a bit darker and more mature, while at least I respect the franchise for trying something new. However great swathes of the show are dull and you don't have to be me to think so. It's got too many characters (nearly fifty) and half the time we're just watching them training. The show thinks it needs action scenes, so we get way too many fight scenes that are either mock training battles (don't care) or unimpressive shootouts against featureless generic robots (don't care).
Mind you, I'm not going to complain about the show's focus shifting from Nanoha to the newbies. Less Nanoha is always a plus... if you're me. There are people who loudly disagree.
What's good, though? There is good stuff. Let's look at it.
Ep.8 is the first dramatically meaningful episode, after which ep.9 is the first good one. (Ep.10 takes us back to normal, though.) Teana has a problem with Nanoha's teaching methods and I was strongly agreeing with her. (What's more, the season does that again later. Unpleasant or villainous people will often have a disconcertingly good point, be they Regius or Jail Scaglietti.) After that, ep.11 gives us a fight that's neither a training battle nor boring drones, which is a big step forwards.
Lieutenant General Regius is an excellent hate figure. The guy's a repellent cock and you'll be keen to see him get his comeuppance. That said, though, you'd think his hostility towards Mobile Section 6 was leading to political intrigue... but no. Not enough story space for that. Let's go watch more training!
Ep.17 kick-starts the real action, obviously. What comes after that has definite high points, with a few characters actually managing to find emotional resonance and even be cool. I liked Fate, Subaru, Vita, Signum and Agito. There's a decent amount of worthwhile stuff in the last few episodes. I was disappointed in at least one case that the show chickened out of killing a hero, though.
In short, it's terrible. It's the work of a non-writer. It's the kind of thing you show writers as an example of what not to do. It's boring, its cast is too big and it almost never gives anyone anything meaningful to do. However it's watchable in a hollow ST:TNG way and it does manage to stumble by accident on a few good bits.