I also dislike "howdy" (and "g'day"), but I wouldn't necessarily go to "grating". Depends on my mood.

I think the issue partially stems from the fact that the internet can be seen as culturally removed. So if I visit someone in the US I expect to encounter their culture, and I am prepared for it. Same for Australia. However, on the internet I expect something more culturally neutral like "hello".

But then again, maybe it's just that certain words/phrases/accents annoy certain people, and you can't please all the people all the time