Why does Buster not join in the family chicken dance?

I don't know if there's a definitive reason why Buster doesn't do a chicken dance over the course of the show, but there are a few possibilities that could make sense depending on how much you want to read between the lines. Your thought concerning GOB using his dance to intimidate Buster is certainly something to consider. It's probably also important to note that Buster's response in that episode was that "chickens don't clap," typical of Buster's approach to coping with many Bluthian activities. This very literal perception of GOB's dance reminds us how Buster sees the world, eschewing things that don't generally make any sense to him.

I also wonder if it might not be fruitful to think about the constant doubt surrounding Buster's lineage -- that is, as yet another sign that he's not full Bluth (of course, Tobias isn't a Bluth, either, but him performing his own Chicken Dance is perfectly consistent with his playing parts to achieve his desire to be fully accepted of the groups he associates with). Or along the same lines, perhaps the lack of the Chicken Dance is more a demonstration of the lack of George's presence in Buster's life as a father figure.

Or maybe Buster is just too afraid (too chicken ... ha ha) to do it himself. Of course, this is all just speculation, and maybe Buster will bust out his own dance in the first episode of the new season.

One interesting thing is that at last year's reunion, when Portia de Rossi and Will Arnett started doing their dances at the request of an audience member, Tony Hale started to clap along (http://youtu.be/gTsu-ks9Gb8) -- maybe chickens do clap after all?