The structure of this film is all wrong. It starts off with a repeat of Spock's funeral from Part II, and the problem is, nothing even comes close to matching that for emotional impact - it's downhill all the way, effectively. The show is largely a three-legged stool, with logical Spock, emotional McCoy, and Kirk trying to strike a balance: kick away one of those legs, and what you get is ludicrous heroics, such as Kirk hijacking the Enterprise to go look for his Science Officer. Even the central plot is fairly flaky; it's never quite explained how Spock gets resurrected (if that's a spoiler, I'd better also mention the fall of the Berlin Wall. How was the dark side of the Moon?) or why Bones is undergoing somethat that would have got him burned at the stake in Salem.
On the plus side, all the actors are obviously so comfortable with their roles that it's impossible not to be drawn along by them, at least moderately. Spoc...sorry, Nimoy's hand as a director is likely also too heavy, especially for a plot which ends with a fist fight to the death between Shatner and Klingon captain Lloyd (combined age when movie released: 98 years. They should really know better) on a conveniently-exploding planet. In many ways, it's a nostalgic throwback to the original series, all it needs is some green-skinned alien floozy to ask, "What is this Earth thing called kissing?" and we'd be there.