Minimize how many times your logo accompanies your email, if at all. It comes across as a heavy marketing campaign and it gets in the way of the real message. People know you are there and don't want to see gobs of pictures or logos. If it's necessary, then keep them small and out of the way of the content. Just like there are messages at the bottom of emails now a days that state "think about the environment before printing this email". It's exactly the same with how much info and how large the email is that's sent to people. Even though people have "unlimited" space in an email account. A lot of people have a lot of email and information to go through and it needs to be kept to absolute minimums. It preserves the "digital environment" in the same context.
Keep emails short and simple.
Have two distinct common signatures, one for new emails and one for reply emails where people are more familiar with you.
New emails will have your full signature content.
Reply emails will have just the necessary signature content for more familiar folks.
If you want to have items stand out, then only go so far as using bold and italic.
Only use increased fonts sparingly, when organization of a long email required.
Reduce email content to skim and scan techniques. If you need to have a long note, it's best to include it as an attachment as a PDF. PDF is a common format that any computer can read and not just isolated to a Word Doc, which everyone might not have. Also, if you use fonts in the document that other people don't have, then the PDF embeds them and the fonts are required to be on their computer.
Make youre email suitable for text based emails or the mobile phone. It's coming up more and more.
Do not change topics frequently in emails. People are ok receiving multiple emails (within reason) if it can help them to organize their email by the single topic.