From the FAQ at http://ycombinator.com/newsfaq.html

    How are stories ranked? 

    On the front page, by points divided by a power
    of the time since they were submitted. Comments
    in comment threads are ranked the same way. 

There are many tweaks that have been suggested, and some have been tried. It's certainly true that bad timing can lead to submissions and comments sinking without trace, but in general the feeling is that the existing system works well enough and with few surprises.