You can still do this. Look for topics here:
http://wordpress.com/#!/read/topics/
and to find a specific topic, just put a forward slash on the end, add the keyword, and put another forward slash, like this:
http://wordpress.com/#!/read/topic/kittens/
The page to view these things has changed and yes they’ve made the topics page look different but this is a site-wide change and it’s unlikely that they’ll change it back. Give it time and they’ll probably think of something else that we might ‘enjoy’ and do it differently again! Everything here on WordPress.com has a habit of changing.
Oh – and don’t shoot the messenger, I’m just a volunteer here. :)