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I was going to write an actual post about numbers and the top 5 specs for each class from data mined from the top 20 PvE guilds in the world, but instead, I'm going to just put this all down so I don't have to type it over and over and over again.
But I have to hand out my email!
Once. From the Real ID FAQ, yes, you have to hand out your email to someone you know in order for them to real-ID-friend you. (There really needs to be a convenient way to verb that.) Furthermore, it's not automatic friending, so if you see a name you don't recognize trying to friend you? Deny the request. The other person is never notified of that. Got an asshole ex trying to friend you on Real ID? Deny the request. Got someone trying to friend you that you just plain don't want to know the identity of all your alts? Deny the request. If someone puts in an incorrect email address, or their request is declined, the requester is never notified. If you have a friend who constantly has keyloggers or has been hacked or what-have-you, don't give that person your email address to begin with. Or make them give you their email address. It's common sense. Why don't more people have it?
But then my email address is saved in their client forever and ever!
Incorrect. From the Real ID FAQ, mutual Real ID friends (and their Real ID friends) will see your first and last name, as well as what character you're on, what zone you're in, and the special Real ID stuff like status messages and presence. It explicitly states in the FAQ that your Battle.net email address is not displayed in the Real ID friends list.
But you said friends of friends! HAX OMG
Shut the hell up. From the Real ID FAQ (do you see a trend yet?), yes, when you click on a friend, you can see the names of their friends, so if you have mutual acquaintances, you can quickly friend them. However, since your battle.net email is never displayed in the Real ID friends list, you don't know your friend-of-friend's email, and they don't know yours. You don't have to hand out your email, they don't get to know your email, and it's easy to make friends. If you don't know who this person is who's trying to friend you? Deny the request.
But what if they harass me?!
I bet you see where this is going. From the Real ID FAQ, not only can you contact a GM - just like if any other player was harassing you - you can block individuals from communicating with you anywhere on battle.net.
But even after you assuaged all my fears, I still don't want to use it!
What, do you think there's more to it than that? If you don't want to use Real ID, don't approve any Real ID friend requests. If you opt in then later decide you want to opt out, remove all Real ID friends from your lists. Friendship is a two-way street on Real ID (unlike WoW in general), and if you remove someone, they don't see you anymore.
If all else fails, you can turn on Parental Controls. They'll be updated to disallow Real ID.
The TL;DR Version
People you are Real ID friends with do see your first and last name (the ones set up as your primary contact in battle.net).
People you are Real ID friends with do see your character, realm, and zone.
People you are Real ID friends with do see what game you are playing - WoW and Starcraft II support Real ID right now. You can safely assume Diablo III will as well.
People who you would like to be Real ID friends do need your email address once, or you will need theirs once.
People you are Real ID friends with do not see your email address in their friends list, nor do you see theirs.
Friends of people you are Real ID friends with do not see your email address in their friends-of-friends list, nor do you see theirs.
- Read the FAQ before you start freaking out next time, internet.
Worst case scenario: someone finds out your real name, and it turns out it's your boss, and he notices both of you are signed on during work. Whoops!
Let's stop the pointless panic and the rumormongering, folks. Let's save the panic for actual crisis scenarios, when things are actually wrong, and stop the "ZOMG HORRIBLE IDEA" righteous indignation over Real ID.