Well, if you show enough of the comments to be useful, then the front page will be really long and cluttered. If you only show the first 100ish chars like gmail does, then it probably won't be enough to be useful, since HN comments tend to be long and well written.

So I guess I'm trying to say I like it the way it is. It would take just as long to read mouseover/preview text as it does to open the comments in a new tab and scan them.