Outlook is the only email application I know of with the absolutely critical feature that Gmail most urgently needs.
In Outlook I can edit the subject line of messages I've received*. (You can edit the body and attachments of received email as well; that's very nice but not essential.)
Neither can other email packages, but the problem is more severe in Gmail because it threads conversations by subject line. Since most humans are still living in the 20th century they don't use intelligent subject lines; important messages get lost in the same-subject-line thread. To add insult to injury, Google's threading model discourages intelligent subject lines.
21st century people know subject lines are critically important. We don't do folders, we do search. The initial presentation for a search result always includes the subject line -- it tells us what's important.
(Digression. I do find it a bit odd that Googlers evidently don't do search.)
If all my correspondents were 21st century I wouldn't have as dire a need to edit the subject lines of their messages, but even so what I consider important may differ from their opinion. I'd still like to be able to edit their subject line on occasion. (Note: Emily, you do fabulous subject lines. I'd say that even if you weren't my wife.)
Sure, this breaks the evidence chain of email. I don't give a damn. I have zero interest in preserving the email I receive in some kind of pristine state. When I archive it I'm doing it for my own benefit, not for anyone else's benefit.
Google, you can fix this. It will help break your compulsion to thread conversations by string matching the subject line (which also breaks Google Groups, but that's another story).
* It's amazing how many people don't know this. Just click on the subject line of an email you're received and type. Shocked?