Sigh, not surprised about gmail, its amazing how many .com's and .org's use it now. I'm not at ALL a fan of how the web client handles a bunch of stuff like replies, & hiding your own messages from yourself.
oh and my biggest gripe, gmail seems to ignore the References header AND replies often don't seem to have that header, so if you're using a traditional threaded email client like Thunderbird, replies from gmail users don't seem to get threaded correctly and show up as new threads. gmail itself seems to thread purely based on Subject.
If people just use the reply feature in gmail it does set the References header properly. It breaks threads intentionally if people edit the subject (which you have to click a separate button to do). But if people hit the reply buttons, the headers are correct. (I did quite a bit of analysis on that when building the new list archives system a while ago). Many other MUAs have much bigger problems -- but even Outlook and the Microsoft web based ones do a decent job today. They certainly didn't use to do that...
And it's internal threading is way more complex than just using subject. But the inability to override how it does that is definitely one of my biggest gripes about gmail. But that only affects the user of gmail, not the others on the list. (Unlike what broken references headers would do)
