"She can be so raucous and down one song, and then break your heart with her subtlety and finesse the next," said Bonnie Raitt. "As raw as Etta is, there's a great intelligence and wisdom in her singing." (Source)
Bonnie Raitt's review of James' career as a singer pretty much sums it up: James packed a punch in every performance, bringing emotion to even the simplest set of lines and making the potentially raucous into something totally over-the-top.
This tendency is controlled, but still noticeable, in "At Last," which she performs closer to a classic blues ballad than a rollicking soul or rock and roll number. The power within her performance of "At Last" is not necessarily the lively and accessible quality of the studio recording, but her lasting ability to perform the song with wit and emotion.