In 3.4 if you are composing a post in a topic and someone posts it alerts you, and you can view it to see if you need to alter your reply.

What happened to this feature?

It's still there.

Yes, this works for me too - an AJAX link is provided at the bottom of the screen which refreshes the thread while keeping your post content intact. Except when you make the post it doesn't automatically show you the post that was made in the meantime, I have to refresh for that. 

Happened again on this site where it did not work.

Oh well. I don't know why. I did have two tabs open, and clicked into the new box and my post was already in the box from the other tab, if that matters. But I posted it and there were new replies and no alert.

I get a little alert (It's actually really nice looking on IPB4) when someone has posted above me before I finish posting. It doesn't quite load as smoothly though when you hit show like it does on 3.X