The "reply" button has been replaced by the "Quote" button (which actually is more accurate).
It operates similarly but it is a little different if you try it. It assists when you want to quote multiple posts.
Basically you can run down the posts you want to include as quotes by clicking the quote button on each of them. Then a Quote push button appears below your typing area. If you press that all the quotes appear for you to use in your message.
Again, the same as the reply button was but operates slightly differently.