Yes I too thought Jack Lord was far too stiff and serious - too similar to the character of Spock.
I think Lord would have been more suited to say Picard, Pike or the Commander from Space 1999.
Kirk's too passionate for Lord. But then again they might have changed the writing to suit Lord more than Shatner.
Random thought; with everything TNG took from unused Cage and Phase 2 content, I wonder if Picard was spun from Pike phonetically.