There have been some pretty high profile defections of people that have felt that HN has changed too much so their is some attrition from some of the old members who felt that HN represented an elite club. Some say the quality is worse, some say it is just different in either case the focus has definitely changed.
In my personally opinion what made me decide to join HN was the self policing of memes and humor, while I enjoy those things they destroy some of the other site that had a serious focus on technology and the industry. Also and most important, HN discourages the "mine is bigger" arguments. To me, that is the one element of a site like Slashdot or HN that will kill it dead for smart individuals, it turns them off and makes them look for the exit quickly.
I think so long as that self policing remains, people will come and go and the focus will change, but smart people will always be attracted to it.
Based on my observations since the Usenet days, over time discussion boards/lists tend to attract individuals who are generally more inclined on average to enter discussions than those who formed the initial community.
Incentivizing posts via reward points, etc. probably enhances this tendency -- the regularity with which discussion of the karma threshold for downvotes occurs is perhaps symptomatic of this.
Finally, long time users are more likely to leave because of changes such as the hiding of comment scores or the shift in topics or the shift in comment quality than a new user who experiences no change.
http://jacquesmattheij.com/attitude+does+not+scale
His point about small town versus large city is wonderful.