Berserk completely converts damage to magic damage so you cannot steal life/mana with Berserk, even with life tap.
You don't take damage from Iron Maiden and Thorns if you just go "Berserk". Because they only reflect physical damage, not magical.
For the stated duration afterwards your defense is zero. You also do not receive the Iron Skin or Dexterity bonus. You can still block as if you are using a shield, however. A defense of 0 will let you block all the time, thus quite often hinder you from doing anything else. Consider not wearing a shield.
Lore[]
There is a fine line between passion and rage, something the Barbarian warrior knows well. A Barbarian must learn to tread this line while separating one from the other and drawing strength from both. One of the most powerful combat skills a Barbarian can learn is to cross that line into rage, expending the sum of his energy and slaying everything without regard for consequences. When you have slain all of your enemies, what is left to fear?[1]
References[]
↑Diablo II Manual, page 65. Blizzard Entertainment, 2000.