It helps if you can watch the series (any series, actually) in the context of their time. Never judge a show filmed in 1967 with 2013 eyes. Nothing is the same anymore, with nearly 50 years of evolution in acting styles, effects technology and storytelling styles. Considering the hundreds of great TV series which have been forgotten in that time, the fact that Trek continues to be seen is something of a minor miracle.
If you every look at other shows of the era, you'll notice the acting styles to be similar. Shatner was always pretty bold, but a lot of actors were then. Especially the Canadians (I say this because Lorne Green and John Colicos popped into my head).
In the sequel series, only Avery Brooks captured that theatricality.
Good on you, though, for giving it another shot. I'm thrilled you're enjoying it. Welcome to the club!