Yes; I do this to discourage people from making too many such posts, and making them too long. I don't want HN to turn into a blogging platform. Also, I wanted to make it easy to tell these posts from comments. If I made the text black I'd have to make it smaller, and then it would look just like a comment.
But I'm not wedded to the current way of doing things. I'd be open to alternatives.
It seems like this system punishes me, the reader, for something that the poster did wrong. If the poster is supposed to write his long rants on his blog and then link them here, the software should tell him. It shouldn't subtly make his stuff harder for me to read, because if I'm interested, I'm going to push through the low contrast text anyway.
Anyway, if you want to nurture the community, I don't think it's a bad idea to make the feedback deliberate. "This is too long, don't post it," will be a lot more effective than changing the color of the text.
Well, the lighter text does punish the original poster, indirectly. It lowers the likelihood of engagement by an audience and I like that subtle approach more than heavy-handed alert warnings. Sites that have that kind of approach feel like I'm in the principal's office and being scolded for no good reason. Subtle encouragement to do other types of behavior feels more like I'm in a hippie montessori school instead - we're not going to come down with the stick if you behave a certain way, but we're not going to give you the carrot unless you do better.
The fact that people are complaining about not being able to read longer blocks of text means it's working just fine. And it's a nice subtle touch - I never consciously considered why I tend to ignore rambling posts and stick to the discussion area instead until now.
But I'm not wedded to the current way of doing things. I'd be open to alternatives.