Ad Hominem

What really IS an Ad Hominem?  According to the Merriam-Webster dictionary, it is:

1:  appealing to feelings or prejudices rather than intellect
2:  marked by or being an attack on an opponent's character rather than by an answer to the contentions made

Is it always wrong to use it in a discussion, and/or when presenting a reply to someone with whom you disagree?

Looking at the above definition, it seems to me that character traits are what is being attacked with ad hominems.  Is it always wrong to attack character traits?  Peter Boghossian in his book "Manual for Creating Atheists" differentiates between immutable (unalterable) and mutable (changeable) characteristics, and implies that challenging mutable (changeable) characteristics is permissible.   Following are examples of character terms commonly used in discussions involving opinion differences:

Immutable by definition

Race
Sex
Country of birth

Immutable by definition, and slurs as used

Idiot
Moron
Cretin

Crude Prejoratives

Dumbass
Shithead
Asshole
Head-up-his-ass
Jerk

Mutable

Ignorant (lacking knowledge)
Gullible (easily persuaded to believe something; credulous)
Superstitious (showing ignorance of the laws of nature and faith in magic or chance; irrational)

While I agree that it is always best to challenge the idea rather than the person, in the proper context, I see little problem with using mutable terms such as ignorant, gullible, deluded, not understanding (blank), etc. in describing a person with whom you disagree if you think it will aid in self-reflection.

http://en.wikipedia.org/wiki/File:Graham%27s_Hierarchy_of_Disagreement1.svg