Here's the problem. I am a 'source material is god' type of person. That's generally because the source material for most series is closest to the author's intentions with his/her series.
Let's take Touma for example. In the anime, he's a decent character. When I watched the anime, he drew me in into it and kept me going but I did not place too much investment into his character development as the anime was not showing his character development too well, instead, opting for him to be more static. This paints an impression in most people as most people will generally not read the source material.
Therein lies the problem. In the novels, Touma is a very well-developed and highly dynamic character. For example, his character development in the Index Arc was about him overcoming his own self-defeating attitude (which piled up due to numerous incidents in his past such as incidents in his childhood and things like Mitsuari Ayu's suicide) and saving a person who he does not want to lose a connection to. It's a very selfish motive that is developed and solidified throughout the volume and culminates when he decides to do everything in his power to save Index.
This part of his character and his development is not translated as well. Small, but important, details were never translated over to the anime. Things like the reason why Touma did not tell Index about her hood still being at his house, things like the reason why Index came back to retrieve the hood, and things like the statement that is repeated countless times in the narration of the volume, all of which could have done wonders for Touma's character development were omitted. It's a damn shame since these sort of details were what made Touma such a well-written character in the novels.
I could go on and on about this but I won't.
Let me touch into something different. Remember upside-down-sensei, the guy floating in the tube? What was his name? Aleister Crowley. This guy is based off the man behind the belief system known as Thelema and a lot of this series has a huge connection to that system such as 'The Book of the Law', Aiwass's appearance, and the mention of the Aeons of Osiris and Horus.
The central tenet of Thelema, "Do what thou wilt shall be the whole of the Law", is actually a very centralized theme throughout the entire series and can be shown to have been applied to the developments of all of the main characters: Touma, Accelerator, and Hamazura. As to how...I'm not giving it away that easily. You should have some fun connecting the dots on your own.
As you can see, the anime had a lot of shortcomings as an adaptation.
While a novel isn't on a time crunch to be read, it is on a time crunch to be produced. Though, given how much Kamachi writes (for the past two or three years, he's been churning out a new volume a month), I wonder if he even actually feels the time crunch.
Ehh...Railgun had fillers...while the original Railgun I can...somewhat...excuse as they did not have enough source material to work with at times, Railgun S had no excuse because there was an ample amount of source material at that point in time...
That being said...I'm still waiting on Index Season 3. There are 20+ Light novel volumes of Index that remain unadapted (excluding the side story volumes like the Kanzaki SS and the Necessarius SS). Some day...some day.
But for now, New Testament Volume 14 hype~