I watched TUC last night, for the first time in maybe 10 years. I didn't find it an enjoyable experience. From the over done racism, to several cheesy moments (Scotty nearly messing his incontinence pants when the Enterprise starts her mission), through Valeris making the main characters look stupid. The story itself felt hamfisted and clunky. Even the Shakespeare felt overdone.
20 years ago, this was my favorite Trek movie, now I seriously dislike it. Why has it aged so badly?
I think it hasn't aged as well because in large part, the world grew up a bit in the meantime. Most of us look at that movie, see the obvious racism, and cringe because that's no longer part of the social norms we find acceptable.
I get that the movie was intended to be an allegory for then-current events, but it was done so clumsily that it wasn't what I, at least, consider good Star Trek. On my personal ranking of the TOS movies, TUC ranks above TFF, but below all the others.
Or it could be argued that this is simply where the social norms of the TOS-universe were at that point. After all, the little bit of 23rd-century stuff we saw in Generations isn't enough to tell us if there was a major shift in society's attitudes (or at least Starfleet's attitudes) toward the negativity we saw in TUC. We don't know if the obvious racist attitudes we saw are ubiquitous at that time or just within Starfleet.
Either way, it's not my cup of tea (no tea-related puns intended).
Add to this the use of the sub-plot of TWOK in this movie (Praxis explodes but doesn't wreck the surface of Kronos with debris, as Ceti Alpha 6's destruction did to Ceti Alpha 5) as a metaphor for Chernobyl and glasnost which was just too heavy-handed for it's own good and (for me) was lacking in the logic department. The crappy production values also are a turn-off (although the last battle was great.) Another thing that gets me about this movie is the plot's idea that the Klingon Empire doesn't have the resources to fix Kronos's atmosphere on its own, and that it would focus all of its energy production on one planet instead of hundreds through its part of the galaxy.
Frankly, I think that the proposed Starfleet Academy movie put forth by Harve Bennett would have been a lot better of a send off for the crew, but that was destroyed by George Takei being upset about being 'replaced' by younger people (he started a campaign to stop the movie, and it sadly succeeded, leaving us with this mediocre mess.)