Jeff Atwood said it might be a preference in this podcast:
Pryor: [...] Are you going to allow that in the settings?
Atwood: [In] this case I actually believe that this is a preference that we will add to the system because you could make the argument that you really don't want to see comments unless they're really really good or maybe not even at all. So you can actually set the threshold at which they get escalated to the question page? Right now the threshold is 0 essentially, the last 5 comments will appear if there are 5 comments, and then..
Atwood: But I felt with comments that it made more sense to escalate them to the page, you have to do it in a way that you're not overwhelming the page with comments, so it's top 5, by date or by voting – if we have enough votes then we'll show you the top 5 by votes.
Pryor: So the threshold is set to 5 by default?
Atwood: The total number threshold is set to 5, we're thinking about adding a threshold variable in that, the comments that are escalated to the main page have to have n votes to even appear at all – in a user preference. Like you could set it to 5 and then the only comments that got voted to 5 or higher would actually be escalated to the main page for you.
But in June of this year, he said:
There was discussion of a comment expansion user preference, back when comment voting was introduced, but nobody seemed that interested in it -- so we haven't followed up.
So I believe that the issue may be considered dead or "status-deferred". They get around to it once enough noise is made.