An interesting feature of the Hearthstone client that I stumbled on last week is the ability to spectate games being played by people on your friends list. It turns out this lets you watch their game, in real time, including being able to see the cards in their hand. It even lets you see their mulligan decision if you start spectating fast enough.
This is a really useful learning tool, for both people. It lets the spectator see what decisions the player is making in terms of cards to play and whatnot. It lets the player ask for suggestions from the spectator without needing to provide extra information about game state. It's especially useful when using an external voice chat program like Skype.
I used this one time with Sceadeau as he asked me for some quick tips on arenaing. Being forced to explain why I would do what I do certainly helped him out, but it also helps me out too because it gives me a chance to figure out why I do what I do. Being questioned gives me a chance to reinforce my choices as right ones, or it gives me an opportunity to fix a leak in my game I didn't know existed. Win-win!
I don't have many Battle.net friends after my adventures with being hacked late last year. My tag is Ziggyny#1233 if anyone wants to add me.