I'm an Aspie, but I was never a Sonic fan since my first exposure to video games was by way of N64 and GBA. (I did pick up Sonic Colors about a year back to see what all the hype was, I actually really liked what I played but I haven't finished yet.)
But I am an MLP sperg, and I think one of the reasons I like MLP can also explain kind of what the attraction is to Sonic.
I don't watch "adult" shows except Arrested Development and Seinfeld, and a big part of that is because of something that a lot of autistics tend to have at least in part: face blindness. I legitimately have trouble keeping multiple characters straight in live-action TV shows and movies because I can't distinguish them, and this detracts from my enjoyment of them because I can't just sit and enjoy, I have to ask whoever's with me "who's that guy again?" or whatever and it just results in me getting unnecessarily confused. By comparison, MLP and Sonic are both super-colorful, cartoonish and feature animals instead of people - nobody who was paying attention to a Sonic game or an MLP episode would ever have trouble keeping the characters straight from one another, when the designs are very much intended to differentiate them. Especially given that lighting changes are never an issue in cartoons like that. It's easy to just watch an MLP ep/play a Sonic game without having to worry about trying to keep characters straight, which is how both those things differ from more mature series that depict humans that can be hard to distinguish from one another for an autist.
I also think that animal-based media in general tends to attract more autistics than otherwise. I mean, anyone who knows anything about Temple Grandin knows that her contributions to society took the form of understanding how cows might be thinking and the problems they might be facing when people are herding them around man-made things. I think it's legitimately easier for autistic people to connect to animals - see: Chris caring more about Patti than anyone else, except perhaps when Bob died; see: my best friend who has mild Asperger's really hating interacting with people but having a ton of animals; see: my feeling like my now-deceased cat was my friend. I think autistics find it so hard to relate to people that it's easier to relate to animals, because animals never hide their feelings the way humans do. Social norms are the most obvious thing that autistics fail to grasp because a lot of the time they're counter-intuitive (why the hell do we ask "How are you?" if we don't expect an honest answer?) when autistics tend toward honesty. Animals are honest. And even if they act like humans, I think that autistics still kind of feel the connection to them moreso than humans. At any rate, it might explain the excessive number of spergs in the Sonic, MLP, Pokémon, and Warrior Cats communities (Chris being the former three; myself being the latter three; my best friend being the latter two - we actually met over a Warrior Cats forum).
Sonic I think is especially a symbol for autistics because of what people mentioned before: he's "cool" and yet at the same time he does not go against autistics' tendency to follow the rules to the letter. I think there's a similar reverence in the MLP community for Rainbow Dash, who is essentially a female pegasus version of Sonic. I actually kind of look up to Rainbow Dash - despite being a 19-year-old girl who's not super autistic either - because she kind of represents the personality I wish I could be: bold, brave, fearless, and above all not shunned by society, and I think the same thing might apply with Sonic to some autistics who don't have any friends but desire them.
I think the fact that Sonic's whole gimmick is "gotta go fast" is extremely appealing, though for what reason I couldn't tell you.
