For example, off the top of my head I can't recall a reading single sneery comment about unattractive women (and I also never observed them getting ignored at meetups,

I've seen Roissy recommended as though anyone would be delighted by reading him. He's a very sneery fellow. On the other hand that was a year or more ago, which is several eternities in internet time.

On the other hand, the perceived friendliness issues are ongoing.

A possibly useful distinction: courtesy vs. cordiality. Courtesy means a moderate level of respect which includes saying "I disagree" rather than "You're wrong". Cordiality means giving positive indications of liking.

They blur into each other. I would read "Thank you" as mostly courteous and "Thank you!" as cordial.

I can sympathize with your desire to not offend-- I'm like that about race. However, reading is cheaper than making mistakes in public, and can be fairly educational.