Emotionally deep characters, writing, role-playing, Creepypastas/Horror, Let's Plays, Watching good people play bad Indie Horror Games, Pokemans, Sandbox Games, HBO's "Game of Thrones", AMC's "The Walking Dead", Star Wars (Legends), Fantasy, Science Fiction, Science Fantasy, Esoteric Philosophy, and Occult Studies.
But without reading the manga you can still find the anime enjoyable, as I have. That's why I always insist on watching the anime before I go read LNs or manga, because then you see the LNs/manga as a more detailed source, rather than just seeing the anime as stupid and inaccurate.
It's not a problem of just being an adaptation. It fails as a part of ToAru. Look at the most recent post on the ToAru Majutsu no Index thread. I detail it there.