A lot of the items on this list are a bit petty considering the more insanely irritating things that happened on BTVS, but some are valid:
Everyone was white. The only non-white characters to ever feature somewhat prominently on the show were black. And you know what? It's great that they went for a little diversity there. But I just want to point out that in California, Hispanic and Asian minorities each outnumber the blacks. Oh by the by. Asians actually make up about 2/3 of the world population! Yet there wasn't a single major character from either of those racial groups. (As a matter of right I am excluding the Potentials from this conversation because honestly, they decided to have one token representative of each racial group and then the rest were, surprise surprise, white.) If we are to believe that the Slayer can be from anywhere on Earth, statistically there should be a much higher chance that she would be from Asia. Just sayin'.
Riley stayed on the show for far too long. It's no secret that Riley never really had a very strong fan base. Chief among those non-fans was Buffy, and even Riley knew it. The "I'm crazy about Buffy but she doesn't love me" scene was replayed in the "Previously on BTVS..." segment of about 7 episodes. Maybe more. Riley's confession to Xander should've come right at the cusp of their breakup and his departure. Instead, they dragged it out for no apparent reason (maybe Marc Blucas had a contractual minimum number of episodes he had to appear in?). In any case, Riley's stay was irritating because his departure was just so inevitable.
Buffy never got paid for being a Slayer. Actually, the financial issues of many a television protagonist are always something of a mystery to me, so a part of me is glad the writers didn't just ignore this, as so many other shows so unrealistically do. But addressing the issue doesn't mean subject your audience to an awful Doublemeat Palace saga. I know the Summers girls had an absentee father, but when Joyce passed away and Buffy became Dawn's legal guardian, Buffy had actually just turned 20. 20! Legally, wouldn't Dawn have gone to their father? And also, wouldn't he have been providing child support in one form or another? It simply makes no sense that he wouldn't be giving them financial support somehow.
And now to address the other, non-irritating points:
Dawn was 14, but she acted way younger. Many teenagers are volatile blobs of emotion. Add to that the fact that Dawn was literally a blob of energy? Well that just seems like a perfect storm for maniacal outbursts... I don't think many doubt that Dawn was annoying, but you learn to just accept that she isn't going away, and you move on.
No one had siblings. One of the themes of the early seasons was surrogate family. The Core Scoobies were a little family of misfits and outcasts who didn't truly belong anywhere else. Having siblings would not have changed this fundamental underlying theme.
No one questioned why a middle-aged librarian hung out with a group of teenagers for several years. It's weird, and if taken out of context, pretty creepy. But it isn't irritating.
The Bronze let anyone in over the age of five. Forgiven for all the musical guests that appeared there. Also, you couldn't have all the scenes take place at the high school. That would've been even weirder!
Nothing really changed when Dawn appeared. I'm glad and relieved. If they had written it so that things had been substantially different, that would've been an insult to viewers who had invested all this time watching 4 seasons of the show only to learn that some of what they watched didn't happen in the fake monk-created Dawn world.
"OMWF" just doesn't make sense. I'm torn on this one. I wouldn't say it irritates me, but it does baffle me.