If you've watched the first season, then you're in for a treat because Nanoha A's takes everything good about the first season, strips the bad, and improves upon it immensely. Seriously, they could not have made this more perfect... well, perhaps a bit more action at the end, but it's the only minor complaint in a long list of Awesomness.
The series starts up again where the last left off, with Nanoha training her magic and Fate being assisted by Chrono and Yuuno with regards to her actions in the first series. Meanwhile, we get our glimpse of the antagonists, as a mysterious book awakens in a young girl's home. Not only does this new girl, 9 year old Hayate, live alone, but she's been relegated to a wheelchair (she does get some assistence froma mysterious uncle relative, but more on that later).
Unfortunately, a new force is active in Nanoha's hometown, as mysterious girl named Vita homes in on Nanoha and attacks her. The reasons behind Vita's actions, how it connects with Hayate, and the so-called "Book of Darkness" all intertwine in a solid plot with great action that will leave you feeling for both the heroes and "villains." Motivation for the antagonists is a big issue with me, and this season weaves the story well, bringing conflict in, while not skimping on reason or believability. All 13 episodes are used well to tell the story, and not a minute is wasted. Since we have most of the character introductions out of the way from the first season, the second season can focus more on pure story.
And Nanoha continues to show just why she's the best magical girl out there; no whining, but going straight forward and facing the dangers with confidence. Not to mention the kick-ass fight scenes that put every other single magical girl show to shame. In a lot of ways, Nanoha justifies the fan feelings that the show is more shonen with mecha references, rather than magical girl, but fans of any of those will find something to like in this.
Top notch. Not only are there more battles, but they are enhanced in length. There are technically only 3 transformations sequences (2 of Nanoha, 1 of Fate), but other than that, no stock footage. Even the transformation sequences for Nanoha are different. They certainly didn't skimp on the animation, but pulled out all the stops; every other series could learn something here. Other than that, the animation is similar to the first series, just more consistent.
Quite a bit of the music from the first series is back, but it still blends so well with the scenes that it enhances them, rather than overwrites them. There are two vocal tracks that deserve mention, playing in their 4+ minute entirety. The first is "Snow Rain" a haunting, but beautiful melody that's played near the end, that will give you shivers. "Brave Phoenix" isn't quite as good, but if you like seeing numerous heroes ganging up on something with all their best attacks, working together, then you'll get chills listening to this piece when combined with the onscreen action.
Nanoha continues it's focus on character motivations and growth, emphasizing courage under fire, and rising to meet the challenge, rather than retreat in fear. Hayate in particular will warm your heart with her many crowning moments of awesomeness; despite the crappy hand she's been dealt, she doesn't give up, either. And no matter how badly she gets hurt, she doesn't run away, making her a welcome addition. The family she gains also deserve mention for the bond they form with her, how they are changed by her, and the lengths they go to serve her.
Again, the one minor flaw returns in that we have 9-10 year old girls who seem incredibly mature and self-sufficient for their age, but compared to the magical girls we've known that frequently whine, cry, run away, act stupid, or detest their powers, wanting to live a normal life... well, Nanoha is a refreshing and welcome change. There is no cringing over stupidity. It's nice to see that characters can show humanity and growth, without resorting to cliches.
And lastly, I do need to mention Raising Heart(or Raging Heart if you prefer) and Bardiche; intelligent devices that really grow into their own and show why they have just as much character as the mages they serve. Raising Heart in particular has several moments of awesomenes.
There's not much else to add, as I don't want to spoil too much, and it needs to be seen to be believed. If I'm overhyping this, well, you'll see why. A's is considered to be by many the crowning moment of Magical Girl Lyrical Nanoha, and of the magical girl genre in particular. All the new characters are welcome additions, and give much needed fleshing out of the cast and universe, while not overshadowing the returning ones.
As I said in the review of the first series, this is one of those overlooked hidden gems, a diamond in the rough, that you won't regret watching at all no matter what kind of anime fan you may be.