OK, let's take the objections one by one:
1. Dawn was 14 but she acted way younger.
The backstory for the famous "Get out, get out, get out" scene was that everyone she knew was lying to her and keeping things from her, and not just little things - she was not a real human and she was the target of a god who was pursuing her relentlessly. If I was surrounded by a bunch of liars like that, I would have said the same thing. BTW I recall my high school years and there were girls who were just like her.
2. No one had siblings.
I would have missed this because neither my wife nor I have siblings. But I would have written the story the same way because siblings would be a distraction to the storytelling except where they were part of the story like Tara and her brother, Donny. If they don't participate in the plot, they are superfluous and siblings would tend to turn the storytelling into either "The Waltons" or "Married with Children". There is enough relationship angst in BtVS without them.
3. Everyone was white.
The exceptions have been noted and blacks are about 10% of the US population which is maybe a little more than their representation on the show, but more surprising than the scarcity of blacks is that you have a show set in California with no Hispanics. Really? I guess the author of this comment never watched "Friends".
4. No one questioned why a middle-aged librarian hung out with teenagers for several years.
He works in a school. Schools have clubs led by teachers and no one objects to coaches hanging out with athletes or students taking an interest in the library. Also, parents are dysfunctional and the whole town is oblivious to the goings-on at the hellmouth. Why would a teacher with students who were willing to stay with him draw suspicion?
5. Riley stayed on the show far too long.
The way he was written, I agree. A psych graduate student should have been more like Holden Webster than corny corn-fed Iowa farm boy. Also the idea that he was an elite soldier in his twenties who never completed even a simple mission, disobeyed the chain of command and was still recruited back into special ops is just ludicrous. It's not that he stayed too long, he stayed too wrong.
6. The Bronze let anyone in over the age of five.
They refused Anya a drink because she had no ID even though she was 1100 years old. We never actually see anyone five in there - they were mostly students. But damn, I wish we had something like the Bronze near our school!
7. Buffy never got paid for being a slayer.
Good point, but you could never sell the idea that Joyce didn't know about Buffy being a slayer if there was a cheque in the mail every month. And our military doesn't get princely sums for the hazards they face (until they quit the military and become private contractors getting paid for kills, which Buffy tried to do unsuccessfully in Season 6). And the writers loved the idea of Buffy taking an entry-level food services job because a lot of people, including them, remember doing just that.
8. Nothing really changed when Dawn appeared.
Everything changed. Buffy was willing to sacrifice her life for someone who she knew was not real. She ended up being the guardian for Dawn and sacrificed a lot, even when she returned from the dead.
9. Once more with feeling just doesn't make sense.
The only part that doesn't make sense is that klepto Dawn has Sweet's talisman but Xander takes the rap for invoking him. The rest of the episode moves the story along tremendously: Buffy reveals she was in heaven, not hell, the split between Willow and Tara becomes both visible and viable, the prospect of Giles leaving is raised, Spike sings of his obsession with Buffy, the romance with Spike begins with a kiss from Buffy, Xander and Anya show their misgivings about marriage, the disconnection the scoobies feel with each other and most importantly, THEY GOT THE MUSTARD OUT!
