Recently, Jeff was inadvertently alerted to the fact that the rate limit on comments was a flat 30 seconds, and that users could button-mash their way to success. Apparently, this was not the original intent; the original intent was to reset the timer every click, so that you could potentially end up waiting multiple minutes before being able to post a comment, if you were unfortunate enough to mistime and be off by a few seconds.
Admittedly, this is likely more of an issue on Meta than it is on the other sites, as Meta tends to encourage far more discussion.
This change appears to have been implemented very quickly (quickly enough that Jeff was able to edit his comment to indicate that it was complete -- admittedly, he is God as far as the database is concerned, so he may not be constrained by the 5 minute window). From the outside looking in, there is coincidentally a new bug with rate limiting vote count fetches that seems to have popped up in the timeframe for this change.
I'm proposing that this change solves a problem which simply doesn't exist. While it's understood that comments are going to be second-class citizens, I think Jon B voiced the community's opinion quite effectively when he said:
@Jeff - that's really lousy. If I try to post as late as 29 seconds after my last post, you're not just punishing me, you're wasting my time
It's one thing to make us wait 30 seconds. I'm OK with that. It's quite another to make the counter reset like some warped game show from Hell, such that trying to post a comment if you're a fast typist (such as I am) becomes a Sisyphean task.