Avoid its, let, let!, specify, before, and subject.
Really? That cuts out the legs out from under the RSpec DSL. Might as well go back to using test/unit.
As to why they avoid let, Thoughtbot says: "We have a blog post coming out before too long that will hopefully explain." I'll wait with interest for those pearls of wisdom -- but in the meantime, take the style guide with a grain of salt.
EDIT
The aforementioned blog post makes for some interesting reading.
