It is to prevent/reduce the posting of large amounts of text. Big blocks of grey text are hard to read, so it discourages people from posting them.

Apparently it is preferred that if you have that much to say, then you should create a blog post and then submit that.

Text posts are always grayed out here. If you check out the Ask[1] portion of the site, you'll see that all of those are as well. There's no meaning to it, unlike when a comment gets grayed out due to downvotes. It seems like a silly design choice to me, but all of them are that way.

[1] Clickable: http://news.ycombinator.com/ask