Filter didn’t work on content class (hybrid_post_attributes)

For your filter to work, the Hybrid Core code would have to run something like $abcd = apply_filters('hybrid_post_attributes',$something); and I don’t see that in the code you posted. In fact, I don’t see apply_filters anywhere at all. Based entirely on that posted code, you can’t filter those items.

I don’t think you understand how filters work. You cannot hook to a function. You need to hook to something specifically created with apply_filters or do_action (for action hooks). See:

Clarification on filters and hooks
http://codex.wordpress.org/Plugin_API

However, my guess, based on the name, is that hybrid_get_post_class is itself hooked to post_class. You should be able to hook your own function to that.

add_filter('post_class', 'my_post_attributes');
function my_post_attributes($classes){
  $classes[] = ' item';
  return $classes;
}

Here is your “one line” version, @ChipBennett.

add_filter('post_class', 'my_post_attributes');
function my_post_attributes($classes){
  return array_merge($classes,array('item'));
}