Personally, I love email:
- It's still the best way to talk online, overall – the most open format, the best client programs.
- Online beats offline since everything is archived and searchable.
- Written beats spoken since you have time to think stuff through, and you can attach images, spreadsheets, code, etc.
However, I noticed that email discussions bring the worst out of people, whereas walking over to them and talking brings the best out of them. I guess it's because emails feel impersonal, leading to "email rage" much like feeling isolated inside a car leads to "road rage".
On top of that, for many people email is their todo list, there still really being no better alternative for keeping a todo list. What this means though is that sending an email with a suggestion implying work on their part without prior face-to-face discussion looks like a written order to do something. I believe this impression can't be avoided even with the most polite, "pretty please"-infested wording. It still feels like "you didn't even bother to talk to me and you expect me to do things!"
So I decided, roughly, to never open any discussion over email. It's fine for followups and bug reports, and it's fine if it's known to work for the people involved. But my default assumption is that email is an evil thing capable of creating tensions and conflicts out of nowhere. Much better to call the person, check that they're available to talk and go talk to them. Then, maybe, send them the summary over email to get all that archiving and searching goodness without the evil price.