Emails aren't tasks.
In any case, Outlook allows for assigning tasks to emails and organize them.
> Hard to split conversations into separate threads when two topics diverge.
An UI/UX design issue.
If the subject is no longer the same, one could provide an UI feature that allows them to be viewed separately.
> Leaving a conversation isn't just your decision: you have to both reply all and hope that no one else replies all using a previous email in the thread.
Leaving a conversation is my decision. I just stop replying and create a rule to delete all further emails using the same topic from the group of senders.
> In a corporate email, inability to prevent mass emails from other employees. Hard to do for personal email too.
In some emails clients, specially corporate ones, it is possible to customize how send, reply and reply-all work, either via group policies or custom plugins.
GMail has not wanted to, which is quite different from being able to.
