To a first approximation, meanness seems to be the deliberate cause of offense.

I think that that definition captures a lot of meanness. And it probably nails the root cause of meanness.

But, if we're using "meanness" to mean something like "egregious not-niceness", your definition misses something, I think.

If niceness were just the opposite of meanness as I understand your definition, then niceness would be something like "the deliberate implication that someone has or should have high status." But I don't think that that is what Alicorn is talking about.

Here's my attempt to define niceness. I am nice to you if I show that I care about and empathize with your happiness or unhappiness. Furthermore, I must show this in a way that wouldn't normally cause you distress. (This caveat is to rule out things like "tough love", or stalkers who think that their attention makes their stalkees happy.)

That seems to me to be the kind of niceness that greases the wheels of social interaction.