Reddit has them and it make comments sections tolerable. Why doesn't this site use them? It relegates the top comment and its hundreds of child comments to the top without sifting through any other comments.
If that's something you would like then the HN Utility Suite plugin for Firefox, or Hacker News Collapse Comments plugin for Chrome will give you that. (I haven't tried either of those plugins for a while. They may be broken.)
I would do this, however I mostly read this site at work. Also, plugins add additional stress to browser speed which as programmers, you'd think would be avoided at all costs.