Difficult? Not for everyone.
What you don't see is just as important as what you do see.
Just because you come to conclusions easily doesn't make you right. Doesn't make you wrong, but step back and think about it. Are you so stunningly insightful that you know the inner workings of complete strangers better than they know themselves, based on a few forum posts? Does the possibility that you're *not* seeing the whole person occur to you? What you don't see *is* important, like you say... but just because you can't see it doesn't mean that it's not there (especially for an online short-time acquaintance). Obviously we all have opinions about others on the forum as to type, etc., but trying to overrule someone else's self-assessment, stating a high likelihood or certainty? Like I said, a great way to be wrong.
I don't know if you've noticed, but more SJs by FAR than NT, NF, or SP will make statements like "I don't fit any one type." I could go into the whys, but I won't.
Actually, I have NOT noticed this. The SJ's I know do not tend to say such things - they're actually quite confident in who they are, type-wise (and otherwise). What I have noticed is that sometimes, people with strong type preferences (person A) can tend to make assumptions about people who don't have (or do not display) strong preferences (Person B), and in such cases almost always assume that person B must somehow prefer the opposite of what person A does (I've caught myself doing this, and it's why I noted that Falling2Fast's wife may be assuming that he fits into one type simply because she exhibits an opposite type very strongly.)
If I wasn't confident, I wouldn't argue it. Are you really going to tell me that the average ESFJ would be adept at typing herself or anyone else via analysis? I don't think you've thought this through, my friend.

You'll note that I used the word "assessment" and not "analysis." (I did, in fact, think it through.) While "analysis" may or may not be an appropriate word, I'm quite confident that an ESFJ (or anyone else - come on, even intp's can do it

) could learn about type and make decisions about what fits them best (note that I'm most certainly not conceding to your belief that Jen's an ESFJ.)
Y'all should try that continuum system out, and let me know when you realize a continuum system doesn't relate to type classification, and has no practical use.
Just because it might not relate to type classification doesn't make it a) wrong, or b)impractical. Not everyone fits into a type so easily, especially when it comes to introversion/extroversion.
Anyway, I've said my bit. Sorry for the derailment, Falling2Fast, and good night all.