One small pet peeve I have is for very long (casual or advanced-level) posts: dialogue scenes can feel too dislocated. If there's a really long reply with several lines of dialogue addressing different points, it's hard to imagine a scenario in which you can reply to every line.

[hider=For instance]
Player A:
"Le line of dialogue"

Player B:
"Le line."
Le narration.
"Le line."
Le narration.
"Le line."
[/hider]
It really makes dialogue scenes challenging to pace, because you're not sure if your reply is in between each of your partner's lines, or a long series of lines that every reply of yours comes afterwards. It's not a huge problem, but it crops up often enough.