I've done a search and couldn't find another topic on this, but it may have been covered on the old board.
As a bit of background, another messageboard I use is based around a Goth/metal band. There's a wide range of first languages and ages on the board, which I bare in mind, but it can have the worst examples of internet etiquette ever. I actually started a thread on behaviours that could upset / irritate other users but the main offenders obviously did not read this.
While I am quite happy to forgive a typo or misunderstanding the following really raises my blood pressure:

People who ask a question but ignore the answer.
The IRL equivalent would be:
Bobby: Where are my shoes?
Sarah: Under the bed.
Bobby: [moral outrage] WHERE ARE MY SHOES?
Lisa: Er, they're under the bed.
Bobby: Where are my shoes? It's so stupid!

People who don't read a thread before replying. I'm sorry, but why should we value your opinion when you're too lazy to read anyone else's.
IRL equivalent:
Bobby: Where are my shoes?
Sarah: Under the bed.
Lisa: I think they're under the bed.
John: Have you tried under the bed?
Toni: No, they're on the doormat.
Seb: Under the bed...

People who attention seek (especially with too many photos!) Now I know we all need to vent at times and seeing a photo of someone, especially a nice or funny one, can be entertaining. But once we have seen about 50 of them it gets a bit old.
IRL equivalent:
Sarah:[holding up professionally taken photo after professionally done make-over]Here is my photo.
John: Ooh. You are pretty.
Sarah: No I'm so ugly. Here are five more pictures.
John: I think you are pretty.
Sarah: My life is so sad. I am so fat and everyone is mean to me. Here are seven more photos.

And more of a board specific one. We have an All Your Own Work forum for any art / writing etc we have done. Nothing wrong in this. Except the false praise people pile on. If I ask for constructive criticism saying "ooh it's lovely" when you've not read it is hardly helpful. Equally [insensitivity approaching - warning!] if someone writes terrible Goth poetry and everyone tells them it's great they will continue to broadcast their fictional problems with more and more cliched nonsense.
IRL equivalent:
John: My life is sad, this time is bad, my heart is dark, I'm in the park.
Sarah:[who was on her phone when it was read] Oh! Best poem ever!
Seb:[who knows nothing of the ways of poetry] That was great! Really deep.
John: huzzah! I am the greatest poet alive!
I'm all for niceness, but make it genuine.
What annoys you on-line?