-of moderate or low quality.
-ordinary or so-so.
According to Merriem-Webster dictionary, mediocre can be defined by both of the above. When considering how to review this anime, this is the term that kept flashing before my eyes. After careful deliberation, I can say that this show was not mediocre, but mediocre nonetheless.
Index has a thrilling premise and a vivid universe; many interesting or unique characters are introduced throughout the plot (if you could call it that). Where this anime really falls short is in its execution. Where the first six or seven episodes offer what looks like a rapid roller-coaster ride that will keep you nailed to your seat, it quickly devolves into almost a joke. The many characters who at first seemed so great soon become stagnant and underused. Characters are brought into the story with the tantalizing promise of seeing more of them, only to be unfortunately discarded before the opportunity to witness character development arises. At the same time, some characters who are barely even introduced reappear later on to the utter confusion of those watching. At best I can say I was thrilled by certain characters only to be let down when they failed to show up later.
As far as animation goes, this was a relatively standard or above standard series. Level of animation quality was consistent, although it was never truly a sight to behold or so forth. There were definitely many scenes (usually displaying the city or subterranean lairs of badguys and such) that were eyecatching and even attractive, however this was not an anime that would wow anyone over with visuals alone.
One of the few saving graces for Index is its soundtrack. With a set of catchy techno beats, guitar riff BGMs, and two memorable opening themes, Index had a stellar soundtrack. Although the music didn't exactly fit the feel at times, it was usually very effective in pumping me up during the somewhat dull fights or forcedly emotional conversations. As far as voice acting goes, there were several voices that I found shockingly good (Accelerator, Kuroko, and Misaka) and others that truly were more typical than typical (Touma, our protagonist).
Returning to the concept of mediocrity, I have this to say: by the first definition, Index is quite mediocre. It is, in most senses, of average quality with many substandard points. At the same time, I feel it does not fit the second premise. This series, although I degradate it so, is actually a favorite of mine along with its spin-offs and sequel. In all honesty, I feel it is of low quality and doesn't deserve much praise. Why is this? I can say its mostly because it is not ordinary and so-so. Index manages to craft a world (somewhat shoddily) that is intriguing, mystifying, and addicting. While I may complain about the issues with the characters in Index, I must say that they were probably one of its strongest points. One of the few points I can praise Index on is its ability to generate different opinions, mainly on its characters. I often hear friends argue whether Accelerator, Misaka, Index, Himegami, or a slew of other characters are the best (Kuroko is the best character though).
Ultimately, I can say that this may not be a good anime by a long shot, but it absolutely is a likable and endearing one.