It is more personality, rather than voice. Relying on a sexy voice or whatever is useless because your job is to bring awareness and insight to the game, not to turn on little boys.
Good:
* Not pandering or relying on annoying/cheap conversational pieces, such as memes, standup jokes, or whatever
*Actually commenting on the game, providing deep insight of the situation
*Having your commentary accessible, meaning people actually understand you
Bad:
*Some narcissistic lunatic who thinks he can actually narrate because of he thinks his voice is good or whatever (hint: it's not)
*No one wants to hear you admire your own voice over the mic. Seriously WTF?
*Using extremely repetitive phrases like "step it up" over and over again.
*Being boring and have no substance.
I don't mind about being biased about certain characters as long as the narrator is making it clear that he favors a certain character and is explaining the reasoning behind it. I like someone that takes a stance and can back it up. It opens up discussion.