Whois Registry Database

Security concerns? Not that I can think of. But personal privacy, that's another matter.

If you register a domain name, you are required to provide accurate contact information. But the domain registry (sometimes called the Whois database) is a public resource.

And as such, anyone can query the registry and learn your name, address, phone number and contact email. (Of course there are plenty of other ways people can get most of that information, including the phone book, but I digress.)

Most domain registrars offer to shield or proxy your contact info for a small fee. You WILL get spam, both in your email and in your snail mail box if you register a domain and you do not make it private.

But the only downside for most people is that you'll get more junk mail. Personally, I don't bother with the privacy option, because I know that a determined person will be able to find my contact info one way or another, if they really want to.