The two biggest issues for me have to deal with communicating with groups of people.
First, I hate being involved with an email conversation between several people that lasts for days and you end up with a 100 email long chain. It's difficult to wade through the crap, everyone has their 10 line signature and the useless privacy warning for a 1 line email. It's annoying and very inefficient.
The other annoyance is not being able to unsubscribe yourself from threads that no longer are of interest to you.
This happened recently at work. I was communicating with our QA manager and our clients QA manager. I had taken care of my part, and they continued the conversation through email yet I still received every email because they would just reply all. Eventually I had to send an email and say "next time you send an email, please remove my name from the list unless I actually need to read the contents of the email."
To me, a simple solution to this is a threaded message board (even ones as simple as like vBulletin and the like from the early 2000's, obviously built with better technologies).
With a message board you have control over how much information you consume, but if someone really wants to send you something, they can send you a private message and ask you to join a conversation. You subscribe to threads of conversation, and then unsubscribe when they no longer interest you.
Find a way to do that in an email client and I'd easily pay $100 for that piece of software.