Yeah, it's true about Shatner's acting early Season One. Top notch. All those early eps, really... very dramatic, theatrical, and quite deadly serious drama. Very little light-heartedness. I get the sense that they were making a serious dramatic program about these people and their jobs and oh yeah, it happens to be in space in the future.
It's only later on in the series do we see Shatner lighten up. And of course, then came in the adventure and fun which is a large part of TOS appeal.
But sometimes, I still would have liked to see more eps played in the more serious dramatic tone of those early Season One eps.