Auto-updating has a lot of potential but also a few usability concerns.<p>You'll find on other auto-updating comment systems that "x new comments" is introduced so that you don't lose the spot you were viewing and you have the choice of seeing the new comments or not.<p>Animations and color can be used to show the movement, appearance, and disappearance of items. This would be great for seeing how votes and positions of items are changing.
I like it and I very much like react.js, but there's a very bad flash of no content followed by quick iterative loading, a frequent problem with pure JavaScript applications. I bet your react component's render is calling multiple times for each new item you load, and that seems unnecessary. I realize react says you shouldn't worry about this, but there I doubt that multiple renders are going to be faster than a single render as with each render the CSS has to be recalculated by the browser. When the browser parses pure HTML and renders Hacker News proper, it's not that slow, you do not see each item load.