I think the differences from other-Trek are two-fold: stylistically and narratively and both come in sync by design. In terms of style, the performances are more underplayed than usual, in line with the big 70's austere sci-fi style introduced in 2001 (though I've always been of the impression that that austere style in Kubrick's film is a critique of the dehumanization of the modern society), but not robotic. There's a lot to read into those performances, they merit attention to be rewarded (doesn't the whole TMP?), a lot of nuance, nothing is said in an obvious manner... Shatner, particularly, does a lot of heavy lifting, in little gestures. The way he says "will you sit down" at Spock in their first scene, for example, he's clearly begging for the man to open up... the sign of relief of his face in the "this simple emotion" convo says it all, the evolution of his relationship with Decker, learning to measure himself against him... one just needs to pay attention, if you're up for it. TMP doesn't make it easy, but I'm personally glad it doesn't! This brings me to the story, I'm sure a lot of people were disappointed back in 79 that they didn't get the Kirk & Spock in the big screen that they remembered but the story purposefully put them out of sync... Spock, trying to become a pure Vulcan by purging all emotions only returns to himself after the mind meld with V'Ger, Kirk, an awkward, somewhat paranoid captain lacking confidence after years behind the desk only finds his footing when his crew is whole again and he's forced into a battle of wills against the Illya probe/V'Ger, McCoy, at a lesser extent, called to adventure against his will, takes some time to be his quippy self... Decker is somewhat sufficient, a proto-Ryker lacking some extra charm but he's not robotic at all, just too serious, the rest of the crew don't have much to work with but they're the same, really... the movie is the journey of these characters rediscovering themselves so they can be the crew of old again. A sort of story apt for a TV pilot (wink, wink). Maybe it wasn't what some people wanted but maybe that's a good thing... well, it certainly wasn't for it's sequels' budgets but I like that TMP is its own thing.