The fact that Dorian's plot is neither about his father accepting his homosexuality (his father's issue is that Dorian allows his preferences to take precedence over his duty, not that he has those preferences to begin with) and does not end with the conflict neatly resolved belies it being an "after school special" -- at least, in my opinion.
I am inclined to ask though... what is Dorian's plot truly about then? I think I read somewhere it was about the refusal of Tevinter society to discuss matters of sex and sexuality generally, not so much their animus toward any particular kind of practice and so on....
As far as that goes (if I interpreted that right), generally speaking, if someone is outspoken against something (lets say, homosexuality, in this instance), that generally means they've harbored those thoughts and are not being forthright about them. That's one of those things that happens to be true. However, plenty of people are just as boring as their outward appearance suggests. Some have a lot more interests than sex with the same gender and never talk about it really.
As far as people's dissatisfaction (whoever was dissatisfied) with the story generally, I can't help but think there's this certain feeling in not necessarily just LGBT but this kind of "needing father's approval" (or even society's approval). If Dorian was truly a Tony Stark kind of figure I would of expected him to kind of like build a mini army and sort of take Tevinter society by the throat and force them to spill their dirty secrets. Gay, straight otherwise (heck, lets say Dorian's conflict was about his highly unusual or otherwise iconoclastic girlfriend) I think people would still feel this kind of... slight disappointment perhaps, regardless of the exact issue, that he doesn't have this "screw the man" sort of thing you see in mega iconic characters like Han Solo or something (weird comparison maybe, sorry).
The feeling is disconnect with parents is a common theme, not just for homosexuality but all kinds of things. My parents for example can't stand all kinds of things about me, but I don't personally seek a kind of ultimate reconciliation, I just try and find the common ground and try and do the best thing possible there.
Anyway, I think, matters of sex and sexuality aside, there are things that people might not like about it that I can understand. There are also things that people like about it that I can understand.