Yes definitely. One I have been apprehensive about is the full-on chat plugins for Discourse. I view Discourse as a system of paragraphs, and at least sentences, and chat is anathema to that. So bolting chat on to Discourse makes me very, very nervous.
To be fair we have evolved a few more chat-like features over time such as forms of presence (you can see if someone is currently replying to a topic if you’re idling at the bottom of a topic), native browser pop-up notifications (these even work on mobile for Android), and so on.
The other one that comes to mind is the invitation system (aka getting users to actually visit your Discourse), there was so much interest in sending invites that we ended up enhancing that system considerably over where it was at launch.