Reply vs. Reply All
Gmail's default reply button only ever responds to the single most recent sender, even when a message was originally sent to a longer list of people. That's a sensible default for one-on-one conversations, but on a thread where several people are actively discussing something, using plain Reply instead of Reply All quietly cuts everyone else out of your response, which can lead to confusion, missed decisions, or people asking the same question twice.
Before you click send
Reply All fills in whoever was originally included, which isn't always exactly who should see your specific response. If the thread has grown to include someone who no longer needs updates, or if your reply contains something meant for a smaller subset of the group, it's worth manually removing recipients from the To field before sending rather than assuming the auto-filled list is always correct.
Frequently Asked Questions
Does Reply All include people who were only Cc'd?
Yes -- Reply All includes everyone in both the original To and Cc fields, not just the person who sent the most recent message.
Can I still remove someone after clicking Reply All?
Yes, the recipient fields remain fully editable after choosing Reply All, so you can delete any address you don't want to include before sending.