Ok, I see people on both ends of the spectrum, and for awhile I was thinking I'd withhold an opinion from this thread. But I'll give my 2 cents. 
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[b]Should you read all posts thoroughly[/b]? Yes. What possible reason would you not? (Other than a possible meta-game, which most people above the free roleplay don't do anyway). The reasons why you wouldn't want to are 1) Time it takes to read, 2) Attention span, 3) outside stress, and 4) posting deadlines. 

Now, a lot of people on here who are strict into the 'read all posts intensely' generally have a good grasp on all of these things that keep people from doing it. Not saying it's always easy, but they have a system or a mindset that helps them do it. A system that doesn't necessarily work for other people. Not all have/can do that. [u]Those who don't have that aren't necessarily worse at roleplaying either[/u]. They could be really good at writing (posts), they could have a far better imagination than most, and do well with smaller interactions, just not the best at reading or keeping up with 8 people who also write 16 paragraphs and do 3 collabs in 10 days. [s](Also, some other people in the roleplay could just have shit characters that the GM allowed in for some reason, who do nothing and just confuse people)[/s] I don't think they should feel bad about having those issues. I think they should work on them, definitely. And do as Dervs said, with either trying harder or choosing different roleplays in the future. But I think to say they're bad at roleplaying is just plain elitism. 

However yes, at the end of the day, if you're responsible enough to not meta-game, [u]you should read every post thoroughly[/u]. [u]A roleplay is a shared experience[/u], ideally. Not just your say. However, I will say

[quote=@Dervish] 
Personally, I can't stand one liners and I find really short and detail devoid posts completely uninteresting and it's hard to give a crap about a character if there's no development or personal insight. 
[/quote]
that while I agree with the gist of this, small summaries are very good for NRP's. At least to put at the bottom of a post. Not so that's all they read, but it helps them focus on when they do read a post so they know what army is going where, or what political upheaval has transpired and they don't miss it through a typo or flowery wording. 

If they don't read the entire post, they will miss subtle hints that my King is either a grim faced hard ass, or he is jolly enough to break bread with your faction, and if they miss that then it is their loss (and they deserve the loss). However, at least the narrative will continue easier and they'll know that my army of 10,000 northmen has crossed the border. (And again, it will be an incentive to read because they'll be wondering why they crossed, and what kind of troops are they, and who leads them, etc.)  

Thank you for reading thoroughly :)