Question: what do the experienced directors try to accomplish on a first rehearsal?  In the case of my pantomime, the parts will all be cast only a day or two before the rehearsal.  My plan was to do a table read, and focus on explaining any lines people don't get, answer logistical questions, maybe the start of some character development ideas.

I plan to start blocking on the next rehearsal.

What do the other directors do on the first rehearsal?  

Yeah, what you do sounds pretty average. Usually, I first will describe my vision for the show and introduce the main theme(s). Then we read through and usually I have the actors make note of their questions and then I answer them after each act or at the end. If it is a musical that opens with a song (rather than a dialogue scene), I will teach them that opening song if time permits. This way, I can start by blocking/choreographing the opening of the show at the first rehearsal (I like to go from beginning to end as much as possible). Also, when I direct musicals, I play a recording of the music when we come to it in the reading. And usually I let the actors sing along so they can get a feel for the music.

I included those tips just because I know that some pantomimes have songs.

Ditto what everone else has said.  In addition, I use this as a time for introductions and to put everyone at ease.  Have some refreshments.  Explain expectations and procedures (who to call if you're going to be late, etc), and other logistics.