I almost always include something very informal (Hi <name>,) in an initial email (or in my first reply of a thread) but often leave it out in later replies. I consider it friendly, but once you start taking the email to bits and reply to individual sentences or paragraphs, there really isn't much point. You're already mid-conversation, no need to say hi again.
Ditto. I actually like it when people take an extra couple of seconds to write "Hi <name>," at the beginning of an email conversation. I like it better than when people start off with just "<name>," which I personally find slightly rude, it's like how a teacher or some other authority figure would start off a conversation where they expect they will be telling you off or something. "John, you didn't do your homework did you?"
I don't find it so rude as to really be bothered by it or feel like I have to mention it, but rude enough that I never do it myself.
