One of my biggest pet peeves in text-based RP is actually quite common.

When it comes to original settings/worlds [i]naturally[/i] players will be required to ask the GM(s) questions about the world they are writing/playing in to have the full grasp of everything. Unfortunately, there are people that just feel the need to ask the most trivial of questions, or worse extensively grill the GM for information that is pointless to the overall plot. Just because they are curious. I have both witnessed it as a player and dealt with it as a GM in the past and it is [i]so annoying[/i].

A quick example; when an NPC is from some far flung country or kingdom that has no relevance beyond the reference and yet a player wants paragraphs of detail about said country/kingdom just to satisfy their passing curiosity.

(As I said this is a common pet peeve and others have most likely mentioned it here.)