Can I be your first? I don't like mandatory conversation/threading. Let's meet!
I don't like having 10 insignificant replies to scroll past to get to the important one I know is there. They're initally collapsed and I can expand them at me leisure you say? I already know what I want, don't make me click once to enter the 'conversation', then again at random to try and expand the relevant portion of it.
If I know that the reply I was looking for with the important details got sent today, I don't want the past week's worth of crap to be shown at the same time when I click to view it.
In my experience, a little education of email users into the delights of Reply, Reply-all, Forward, CC and BCC go a long way to a self-regulating 'conversation' feature.
An Ex-Threaded Conversation user.