Personalize Email Marketing
The following is true in most aspects of marketing, but takes another dimension in email marketing: personalize. Considering what we just said about respecting your prospects privacy, you should limit yourself to solely requesting the information you need, their email address. But optionally asking for your subscribers name (or first name), in addition to their email address, is not to intrusive. This will allow you, with most professional email marketing software, to personalize your newsletters, and greet your subscribers by their name.
Email Marketing Gets Personal
How personal your email marketing campaign gets depends mainly on the amount and type of information you have about your newsletter subscribers. In many cases, this information will come from a newsletter sign-up page on yours or a third-party's website.
Remember to respect subscribers' privacy: requesting too much, or too private information may keep prospects away from your newsletter. You will be safe assuming that your information is not useful or unique enough to warrant that level of trust, initially. It is 100% up to you to convince your subscribers otherwise.
First Name Friendly
Used judiciously, addressing your subscribers by their own first name goes perhaps further than most attempts at personalizing your email marketing campaign. 
Caution: much like variable printing made personalization unremarkable for print, email personalization is not only unimpressive, but very often too close to sounding phony. Our recommendation is to limit first-name basis to twice per email newsletter: once in the beginning (as in "Hello, Charles"), and once at the end ("Well Charles, that's all for this newsletter - talk to you next time").
Tone, vocabulary, and brand awareness
When you email a newsletter to a group of people whose first name you do not (yet) know, you can still be personable in tone by opting for casual vs. formal: "Hello, new subscriber" might well work for the first newsletter. If your content is compelling and different enough, you will succeed in inviting your users to share more information about themselves.
Tip: humor is nearly always your way out. Here's a silly example to illustrate the point: "Since we don't yet know your name, we'll call you "Big Daddy" until you update your information". Newsletter readers will either like being called the expression you picked, or will supply their name!
Regardless of the circumstances, always adopt a brand-aware tone and vocabulary: think of the exact same message written on the one hand by Brooks Brothers, and on the other by MTV: both brands and target segments could not be further apart, for the most part, and these differences should be reflected in all marketing collaterals and communications!