The subject line is the first piece of information we see, yet 50% of the time you didn't write it, and even if you did, the conversation quickly evolves to make it irrelevant.
Changing Gmail's subject line on incoming mail
In regular Gmail, it's simply impossible to change it, but with ActiveInbox, you can click the subject line to replace it with a Sub Task that is an actionable instruction.
You see this instead of the subject line whenever you return to the email, or scan through one of your task lists, such as Today, deciding what to do next.
Changing the subject when you reply
This Gmail can do! When you reply to a conversation thread, in the Reply box, click the 'v' arrow in the top left, and select 'Edit Subject'. It'll create a new conversation thread, with the new email subject you enter.