Coming off of A's, the next season of Nanoha had some pretty big expectations to fill, and they tried to do it with 26 episodes instead of 13. They also seemed to have the goal of really expanding the Nanoha universe, fleshing out world details, and in this vein, I think they succeeded. There were a few flaws in this stylistic change, though, and I'll get to them below.
The series biggest change is the fact that Nanoha and her friends are now 10 years older. While most magical girl shows keep their protagonists young, and "lose the magic" if they age at all, Nanoha is unique in that not only do they keep their magic, but they grow stronger in it. They're more magical women than magical girls now, and it's refreshing to see the genre finally break new ground in this regard. And not only are they older, but they are now working full-time for the Time-Space Administration Bureau, a universe spanning peacekeeping organization. Nanoha in particular has elected to become a trainer and develops a repuation as something of a tough, but gentle, drill seargent.
But for more, let's hit the categories.
The animation quality has dipped a bit, but it is still fairly high. This seems to be because of the doubling of the number of episodes, but it doesn't slip enough to be noticable much. Most characters have just one transformation scene for flavor, although "Forwards" (the four new cadet heroes) have two. So again, Nanoha doesn't make use of much stock footage. There are a few things that were fixed or added from the TV series to the DVD edition, but other than that, youll be satisfied.
There are considerably more sound tracks this time around, providing a good atmosphere for each scene, again airing appropriately to complement and enhance the scenes. Special mention has to go with the second opening, which really tugs at your heart strings, especially once you get to that point in the story and understand why. I'm not the best judge of sound and music, but there was nothing glaring in this regard, so I was satisfied.
This is the first split for many people, because StrikerS adds in a TON of new characters. In addition to what we had last season (minus three or four that are relegated to mostly cameos), we get four new magical cadets, a dozen new supporting characters, and nearly two dozen new antagonists; most of which (the cyborgs aka "Numbers") you'll be struggling to remember the names of. StrikerS comes across as an ambitious project and you can tell they had difficulty making all this work.
If you subtract the supporting characters, most of whom are there to serve as millitary personnel and thus don't have much of their own story (although you'll be surprised by a few of them really standing out in brief but memorable moments), then we're still left with quite a cast to develop. Two of the forwards, Teana and Subaru, are developed fairly well, but the other two, Erio and Caro, are left a bit lacking. We get hints as to their backstory and their connection with Fate, but they could have used some more character growth.
Jail Scaglietti is the main villain this time around, and unlike the first two seasons' antagonists, there isn't much in the way of humanity-redeeming traits in him. He's a mad scientist, and that's about it. Still, he does it with such chutzpah, flair, and brilliance, that you can't help but like the guy; you'll find a smile on your lips everytime he's on the screen, hamming it up. A few of his supporting cast of cyborgs that he created and raised do get enough characterization to recognize them, but most sadly go underused. You have to look to the following manga continuation to really see them get developed.
Overall, what we do see is fairly good; and even the main trio of Nanoha, Fate, and Hayate contiunue to get development. The series really tries to accomodate the large cast, but as I said, the ambitious undertaking is a bit lacking in the follow-through. I'd still give it a B for effort, however. The girls are older, but continue to show exactly why they remain the kickass leaders of the magical girl (now woman) world.
Other than the sheer number of characters to keep track of (that will require rewatching to really get a handle on), you'll still find plenty of what made Nanoha great. Good battle sequences, good character development (for those that StrikerS focuses on), and heartwarming moments where we see the vulnerable natures of the women beneath the kickass exterior.
One thing I haven't mentioned yet, is that the creators seemed to have a bit of trouble adapting the pacing from 13 episodes to 26. Early on, the series can seem a bit slow, and there can appear to be a couple of "filler" episodes, even though they focus on character development. Still, some things could have been condensed or not shown, in favor of bringing the main plot more to the fore and using it to develop some of the underused characters. Part of the ambitious nature of the series, is that they seemed to really want to develop the Nanoha universe, and this seemed to detract a bit from the story.
Still, I don't feel it drags things down too much. It's a minor complaint, especially since the last 8 episodes or so really sell the series. It's like going through a decent dinner that perhaps doesn't have all your favorites, and getting a really delicious dessert at the end. It does throw a lot at you, so it will require rewatching to really catch everything and be able to nail down all the characters as well.
So I still recommend the series. If you were a fan of the first two, you will still find a lot here to like, and I guarantee you'll enjoy many of the new concepts as well. Just be aware of the tons of characters and the slower pacing, and you'll find a good sequel to a great series.