Characters may seem superficial at the beginning but they start changing. Akko, the MC, is really really annoying and stupid until episode 7, I guess (it's the one she has exams to do). After that, she starts changing a lot. She's still a bit annoying but you can see how much she's improving. The same happens with the other characters, the show starts showing more about them. I think all characters here are interesting, but Akko's friends are surely the ones I enjoy the most.
The ideas from the OVAs were well adapted and deepened. Diana had the most relevant change in the series, she's not as stuck-up as in the OVAs. Even though she still has that "superior" pose, she helps everyone who asks for her help her and she doesn't bully Akko (her "friends", though, are still the same bullies).
About the story, well, in the first "season" (the 12 first episodes), it's more like a slice of life. Later, they started developing the "main story". It's still an episodic serie but you can clearly see now the "greater" story behind every episode and there's more action.
Art and soundtrack are great, but, well, it's TRIGGER STUDIOS, so, no less would be expected.
This is a very chilling and funny anime. There's also lots of references to other animes and general pop culture (my favorite one so far is the show's parody of Twilight). If you want to have a nice time and some laughs, this is your show.
UPDATE:
Now that I watched everything, I can update this review. I've got nothing much to add but I'll put my last impressions here: it definitely gets more action at the end, characters are really really well-developed (particularly Akko and Diana) and the story is good and well-played. It's a really good show. The last episode, though, was a bit less intense than I expected.
