Long Posts and Godmoding
Long posts are long for typically these reasons: they are introspection heavy and/or they are action-heavy.
Now, this article is not not about introspection-heavy posts, necessarily. What I'm talking about are posts in which characters are doing and saying a lot of different things. If a post is long because it's packed full of introspection and only has, say, one or two actions (dialogue included as an action), it does not present the same issues as a post which is long because it's packed full of things another character can actually respond to.
Let's say that, as a hypothetical example, you're playing in a thread in which a group of characters are out in the wastelands fighting some monsters. It's an action-heavy scene, so there is a lot of doing rather than introspecting. Most of the other players are limiting their actions, but one person writes up a huge monster of a post where their character does the following: makes a couple comments at his teammates, makes a run for an abandoned building, kicks open the door to find some monsters lurking inside, proceeds to decimate the monsters inside it, then runs to an old control panel inside the building to send up a signal before finally turning and heading back out to his teammates and saying some more things to them.
This seems harmless on the surface, and the person doing this most likely doesn't realize what they're doing. It's easy to get wrapped up in your own writing and forget there are other players and characters involved.
However, what this is, is godmoding. Only it's not assuming action. (The player is not assuming your character, say, flinches when theirs raises their hand as though to hit them.)
It's assuming inaction. (That your character did nothing while theirs did all this stuff.)
By doing all those things without giving the other characters a chance to respond, they are assuming that while this character was running around being a badass, the other characters were just standing there and letting him--which may be completely OOC for those characters. What if someone wanted to grab him to hold him back from rushing straight into danger? What if someone wanted to follow along with him and help fight the beasties? Heck, how were they to even respond to the first few things he said? He didn't give any of them a chance to do it, and so assumed they wouldn't, even if it would have been IC for them to do so. Similarly, the goal of RP is to react, advance the plot, and give your partner(s) something to respond to. In the above scenario, what did the RPer give for the other posters to respond to? What an awesome fighter the character is?
And doing that is just as much a case of godmoding as assuming (without permission, that is) that their character will react in a certain way if your character acts in a certain way. (Such as writing that the other character laughed at your character's joke, without asking the other player if their character would do that.)
I find that it's also viewed as more acceptable. People who would not like their characters' actions assumed are completely comfortable assuming the inaction of others. Which, again, is understandable--it's easy to lose yourself in your writing.
But it's also important to remember that there are other people involved in the thread, too.
Why do I bring up long posts instead of short ones? Well, truly, the length doesn't matter. But I find it happens more in long posts than short ones, because as mentioned, long posts are long for two reasons: a lot is happening and/or they're introspection-heavy. There's more room to do this sort of thing in a 700 word post than a 200 word one, and therefore it happens more in longer ones.
Am I saying either is bad? Nope. Just don't forget there are other players involved!
And remember: assuming inaction is the same as assuming action.
Don't forget that there are other players involved, too, and they also want a chance to play! RPing is about cooperation and collaboration, about being part of an ensemble cast. It's not about being the star! Remember to stay in the spirit of RP, which should be about responding to what was posted, advancing the plot, and give the following poster something to respond to.