You notice it when you're reading email, but not so much writing it. Most email clients show just the first 50-75 characters of a message in the inbox view. Write your first line to that limit, and you'll likely get noticed.
It's a tip offered up by web PR consultant Steve Rubel, who notes that an ebook author reached through his own crowded inbox and managed to get a click-through with a first line that grabbed him. Rubel dubs it "tweetifying" of your email's lead sentence, but it's actually about half a 140-character tweet. The idea is the same though — let the character limit guide you to a punchier first impression.
Tip: Tweetify the Lead of Your Emails [The Steve Rubel Stream]