People speak highly of To Aru Majutsu no Index and I don't get why. The action is good(ish), I suppose. There are some character things that happen. There are some plot twists that are... well, there. We get light hearted fun and occasional fan service, and on the other hand the darker parts are also there giving diversity. A variety of superpowers, secretive organizations, recurring characters, and the occasional villain of the day. It has everything a show should have. Everything should have clicked together and made for a fun view.
Perhaps this is the opinion of someone who has watched too many similar shows, but To Aru Majutsu no Index was a decidedly tedious view. Blaming a show for being derivative in anime is easy but wrong, as the world of references and structures in which anime thrives is as much of a strength as weakness. Maybe the way to describe the failure is that this is a show that feels like you've already watched it when viewing it for the first time. You should be enjoying this take in choices of style, but instead it just feels like a random combination of things that should be fun, but were never really enjoyable on their own.
It isn't that the show is particularly bad. The artwork is above average, making up for the generic writing. And yet, To Aru Majutsu no Index meanders in mediocrity. Moments of cool stuff are quickly drowned out by dull standard scenes. It isn't that the superpower thing is done badly, or that anything in particular stands to blame. At the end of the day, this show feels not only forgetable, but each scene is all but forgotten the moment after you see it.
Don't get me wrong, To Aru Majutsu no Index is not boring, nor is it badly structured, nor does it do anything terribly. Really though, this is one of the rare shows where even an obsessive completionist would be hard pressed to watch the second season. Not because the show is bad, but because it is so firmly average in the grand scheme of things. Bad at least will leave a taste in your mouth, this is a show that ends up being bland.
Writing (Story and Characters):
I suppose that if you are new to anime, then the next few paragraphs about the writing are not for you. With a fresh perspective, the writing of To Aru Majutsu no Index might seem cool. I wouldn't know. I've spent thousands of hours watching anime over the years and cannot attest to this one way or the other.
The writing in To Aru Majutsu no Index is a decidedly generic waste of time. It has everything writing should have, but none of the things that capture the mind of anyone who has seen enough anime. The superpowers are cool, but they aren't special in either concept or use. The shady organizations with ominous purposes aren't particularly menacing or clever, and feel like a million others that we've already seen. The only thing even slightly creative is the concept behind the world, yet even that feels somewhat boring.
Now the story isn't all bad. We have a few arcs that are of various quality, exploring different characters to different depths while providing us throwaway plots in order to enjoy the world and combat scenes. While not original, this is hard to mess up unless the artwork is a complete failure (it isn't). There are hints at an underlying story, but it really isn't there (maybe it comes together in the second season... I wouldn't know). We get tastes of what could become excellent, but somehow the story just doesn't get there.
While excusing the plot and waving it off as "it will all become clear in the next season" is a possibility, with the characters no such excuse exists. Most of the cast are stock characters with standard backstories that seem to have been computer generated because of how generic they are. It is supposed to fit the formula of how a show should work, but the most important part of the formulaic cast is that their interactions are automatically at least somewhat interesting, and this holds true for very little of To Aru Majutsu no Index.
Being too average isn't average anymore, it is boring. Average means that there are some bad things and good things make up for them. But if all the bad and the good are standard, then they can't even be called good or bad anymore. To Aru Majutsu no Index manages to take the most beaten path of how to make things interesting, and ends up having the reverse effect. It isn't subpar per se, which somehow makes this a greater disappointment.
Art (Animation and Sound):
Technical execution is important, and on that point To Aru Majutsu no Index manages to score rather well. Artistic flair is somewhat harder to define. Is it still artistic if it has been done to death before? That depends. Superpowers are notoriously hard to make exceptional that way; people give a lot of crap to people who try to innovate, but it is too easy to fall into the absolute standard way of doing things. And here we have a show erring on the side of caution.
Whereas the writing is annoyingly average, the animation manages to do some things very right. The backgrounds are workable, the show sticks to a color palette, the animation isn't too choppy, there isn't any annoying overuse of still shots... and yet, with superpowers, that is not enough. Even the flashiest superpower (railgun) ends up decidedly dull despite good technical execution. Only one moment with the teleporter managed to actually wow me. Still, the character designs are efficient (if a little fan servicey at times) and distinct, and there are no real big technical flaws.
Sound is where To Aru Majutsu no Index could have risen up to be great. The soundtrack is decent enough, the voice acting is somewhat over the top (which is to be expected for a superpower-heavy show), but... excluding the second opening theme, everything just feels perfectly thematic and nothing more. The sound effects could have been something distinct, but instead sound like the average soundbank for these shows. Everything is done right and it is done well, but nothing stands out. Still, above average, if lacking in creativity.
When trying to take average writing and knock it up a notch, the artwork has to take risks that were not taken in To Aru Majutsu no Index. There is the bare minimum of life in the world and everything, while above average, will not wow most viewers. The characters get more life from their voice acting than from the writing, the animation makes up for the story in various ways... but trying to sum it up, the artwork does no more than balance out the writing into something average yet not memorable in the least.
Overall:
This is a show that gets boring fast, and teases the viewer with a possibility that it might get good at a certain point yet never does. To Aru Majutsu no Index has all the makings of a great and enjoyable ride, and yet ends up being horribly, disgustingly, and unfortunately not distinctly average.