The Lead Up:
Spock is dead, Bones has Spock's soul, the Vulcan chick isn't Kirstie Alley anymore and Scotty's weight gain continues unabated. Kirk and crew then proceed back to the Genesis planet so that they can save Kirk's bastard son because he's being threatened by ... Christopher Lloyd?
Why it Nearly Ruined the Movie:
This cameo transcends the space-time continuum that Trek writers regard with such reverence that they screw with it at least once per movie (and 10 times per season). People who watched The Search for Spock on the big screen in 1984 did not see the Klingon villain Kruge, but rather the perpetually stoned cab driver "Reverend" Jim Ignatowski from the recently canceled TV show Taxi. For our under-30 readers, imagine if The Matrix had ended with Neo having to fight Kramer.
And while the makers of The Search for Spock can't be blamed for what came later, it should be noted that things didn't get any better for the later generation, who watched it post 1985 and saw the guy from Back to the Future who liked to say Jiggowatts.
While he's on screen for a large portion of the movie (being the main villain and all), Lloyd's role seems less like a top-billed performance and more like a cameo that just won't end. In trying to pinpoint exactly what it was about Christopher Lloyd's performance that made a horrible movie worse we discovered that he bore a striking resemblance to a stereotypical relative most of us have.
Lloyd is that annoying uncle that comes for Thanksgiving and doesn't leave until Groundhog's Day. While he's there he eats all the food (takes up all the screen time), bullshits about how he used to be in the special ops (pretends he's a barbaric Klingon warrior) even though there's no picture of him where he weighs under 200 pounds (looks like he could scare a 4-year-old). When he finally leaves (gets kicked into a river of molten lava) we feel relieved that we will not have to see him until next Thanksgiving.