Specifics?
Do I really need to spell it out? Dead alien is laid to rest on a planet created by a terraforming device, whilst one of his friends is being driven insane due to the fact that said dead guy's 'soul' resides in his head. All the time said dead guy's corpse has been re-animated and has become a child again and needs the memories from his friend to become the person he used to be. What's not preposterous about that?
Yes I know this is Star Trek, and similar criticisms of the plot can be levelled at several, if not all Trek movies, but this really stretches the suspension of disbelief for me. Does the Genesis planet keep regenerating life after it's formation - the plot certainly suggests this, as Spock was placed there after it’s formation, so why does it not effect David or Saavik or anyone else that goes to the surface? If this 'Genesis wave' regenerates dead cells, why does Spock become a child, why does it just just not simply bring him back to life as he was? Equally the microbes on the torpedo that have evolved at super speed into those slug creatures, why has a similar thing not happened to Spock -he should have evolved into, I dunno, ‘Q’ or something by that rationale. It's all just a bit... silly to me. All my subjective opinion of course.
But... at the same time, I really like this movie, it's very entertaining, and serves as a good sequel to it’s classic predecessor, and as I said earlier, has a superb score, solid acting, good FX, and a high rewatch value. It introduced a lot of new things to the franchise, and had the balls to destroy the Enterprise before it became a predictable thing to happen on the big screen.
I also personally loved Lloyd as Kruge, he brought some sly humour to the role as well as being a pretty ruthless bad guy - at least his motivations were clear and not just the usual bigger ship revenge drivel that dragged down the later movies. And has many have mentioned, the big E theft is one of, if not my favourite scene in all of Star Trek. Overall I rank it over TVH and TFF, and maybe close to on a par with TUC too. I’m a massive TMP fan, so can’t rank it above that and TWOK, but I still like it. A lot.