I just did some research, it appears one of the main differences between INTJs and ISTJs is that INTJs tend to work better with abstract ideas, and are more creative as a result, whereas ISTJs think more in terms of what they can see and feel, and work better with "concrete" ideas. I can definitely relate to this.
The other main difference appears to be that ISTJs are more likely to follow rules and deadlines than INTJs. I can sort of relate to this, but at the same time I don't actually like following rules, and I do it more because I either feel I'm obligated to, or because they provide a concrete structure for me to work with which otherwise wouldn't be there.
But here's what i don't get, aren't aspies technically supposed to think more in rigid, concrete terms? Going by what I've read, you'd think that ISTJ personalities would be more prevalent among aspies than INTJ personalities. Maybe online communities like WP are just more apt to attracting the more-creative, INTJ variety of aspies.