HN has its shortcomings but lack of avatars isn't one of them.
The quality of the postings here is of a way above average level compared with that on other sites that offer a way to discuss links related to programming and surprisingly wide area to the left and right of that.
Avatars are for people that don't want to read, the usernames are more than descriptive enough, and offer everything from mechanical fishes to Daniel B Markhams, so no lack of expressivity there.
The only people that would use avatars (I think) would be those that are normally found in troll territory.
I think that HN really rewards the time you put in to it, by loading up HN with more bells and whistles the quality is not going to go up.
Less = More.
That said, I agree that avatars are not a good idea. I like them on a lot of sites, and I like the idea of HN as a community, but it really is supposed to be all about the words. Let's keep using words.
This site appeals to the same kind of person who can appreciate the utility and straight forwardness of a command line. Conversely, I like to think the site will not appeal to those who might look at a command line and break out into a cold sweat.