For a long while, I felt the same way. It's very possible Hunter could have grown into the role and had more warmth to his projection of the captain. But when I watch those early Shatner episodes, I change my mind. While some of his acting was over the top, he brought a terrific style to Kirk that really worked. There were plenty of rough edges to TOS acting and directing, because of the intense film schedule at hand (too little time from script to filming), and then the whole debacle as Season 2 came to a close (issues with production staff, Gene being absent for Season 3, etc). Sometimes I cringe at Shatner's performances... but overall, I'm still glad he was cast as Kirk.
I always thought that Shatner's overacting hamming took place in the third season rather than the earlier ones!
JB