Chaoslux is right, this is just too cheap and the effect potentially too disruptive as well. It would definitely need to cost more, but even then I think hindering battlecries, spells, and similar effects on top of what is otherwise Taunt is just too much of a hindrance to the opponent.

Wow, the spell becomes a Super Taunt. :o

@pinkamenaislove  
I feel like “Give a friendly minion +2/+8, Taunt, and “Your other friend characters cannot be targeted by spells or Hero Powers” would be a more consistent and intuitive way of writing that.
 
But then, that entire package isn’t worth 4 mana. It’s definitely in the 6 or 7 mana range.
 
Also, on the mechanic note… There isn’t a single effect that will remove Stealth from a minion, except for damage. If you give taunt to a stealth or immune minion, it removes Taunt from the minion, not Stealth. So, giving this buff to a Stealth/Immune minion should remove the Taunt/Aura effects.

@pinkamenaislove  
Yes, though a spell doesn’t originate from the board. That makes sense for battlecry targets though

@ponysopher  
Characters in Hearthstone just mean any and all things in the specified side of the board. To heal all friendly characters would be to heal all of your minions, and yourself.

@pinkamenaislove  
I read “enemy characters” to be things originating from a character

@ponysopher  
It states that enemy characters, which means minions and the enemy hero, must target it, which would effect other things rather than just attacks. Spells or anything that requires targeting, would apply to this Minion, even buffs, or a Battlecry like Voodoo Doctor.

How is the quoted effect substantially different from taunt?