I haven't read any of the other posts yet so forgive any repetition that might occur. I just wanted to add my two cents. 

I agree completely. Shorter opening posts do seem to encourage players to post more quickly and easily. Speaking from my own personal tastes, I'm not to keen on reading a wall of text, however detailed and well written. It's something I wish I [i]could[/i] cope with but I just seem to...turn of a little? It becomes much more of a task than a pastime. Shorter opening posts give people a nice guided entrance to an RP, they let them get straight into it. They can read it quickly which will spark ideas for them that they can immediately begin to form a post around. I've been there, at both ends. A short opening post I can shoot through, I can begin my response immediately and as you say, there is less pressure to match it for size and intricacy. 
On the other hand I've been in those RP's where you love the concept, you make your character and your really happy, you think it's going to be fun. You come to read the opening post and it's like you have to dissect it. You have to really concentrate and soak in everything it's trying to tell you, and whilst I might enjoy it, it just gets tedious unless you are seriously wrapped up in the RP. You have to give people an easy berth into it, once they are hooked you can start to expand on things, get a little more thorough. This is my personal preference at least, something which I'm not really happy about, but it just seems to be the way I respond to things. I absolutely love the concise 3-4 paragraph posts that give you something meaty to sink your teeth into, with one liners I just give up. There isn't any meat on the bone (to continue this poor analogy). With large text-walls? I just turn off a little. 

Frankly, I don't like the amount of effort I have to put in to try and get myself hooked. If that makes sense? when someone creates an RP they are already swimming in the ideas, they are immersed in what they are trying to create. Other people will think 'Hey that's a cool idea' and join in but they aren't yet to the same level as the GM. When massive walls of text are presented it's almost like I say to myself 'This is a cool idea but...I can't be arsed with that' Which is a shame, I think that's why I'm not a very good writer and it shows my inexperience. 

That's my opinion any way. Ironically this came out a lot longer than I intended XD