Man, this book was dull. That's no great fault, though. Unearthed Arcana (Collins, Decker, Noonan, and Redman) is a clearinghouse of alternate rules ideas, and as such isn't the sort of book you read purely for enjoyment. Best case scenario, one of the sections is really interesting and then ends just when it's getting good (i.e. "Invocations," ritual magic that anyone can use, which distinguishes itself from regular magic by having a specific role in the setting).
But even though this book is not especially entertaining, you can make entertaining things out of it. Something like gestalt characters, where you take two classes and combine them into one, opens up a wide range of customization options. It could make for a memorable campaign with Barbarian Bards and Paladin Sorcerers, but without the compromises that multiclassing normally entails. Likewise, changes to the hit point system will affect how you approach combat, the contacts and reputation systems make it more rewarding to engage with the world, and all the various magic system tweaks make magic more powerful (not that it needed it, but you know how it is).
The best way to use this book would probably be to just skim the introductions to the various sections and read only the ones that you think you may want to use. There is a certain boldness in making a book where you know not everything is going to be for everyone, and being selective with your time investment is nothing more than honoring that boldness. Plus, it would have spared me from having to read the "Sanity" section.
Now, I want to tread carefully here and approach what I'm about to say with maximum kindness, because yes, this section was offensive and boring and weirdly somehow both medical and exploitative, but I knew it was going to be that way as soon as I saw that it was a Sanity system. With the possible exception of Unknown Armies 2e, there's nobody that does it well, and I've yet to read one that wasn't the worst part of whatever book it was in, so I don't want to point a finger and say "you did a bad job making this," because the main mistake was simply doing it at all. At least this section didn't call anything a "derangement."
Though they did call being transgender a "psychosexual disorder," which had to have been behind the current psychological consensus even in 2004, and nonetheless ridiculous in a world where the polymorph other spell exists (actually, I went back to check - that spell was only permanent in 3.0 and earlier, with 3.5 they changed the duration to 1 minute per level). However, I'm pretty sure that was mostly just laziness. The sanity system borrows liberally from Call of Cthulhu and it's likely that even early 2000s Call of Cthulhu was just coasting on research originally done in the 1980s. At least, I like to think that. I can't say I was any better about trans issues when I was 22, but I also know that I'd be highly embarrassed to have my bad takes dredged up after all this time.
Also, there's some racism, of that nebulous rpg variety where you make up a fantasy race and then describe it using language that resembles real-world racism. Like, in the year 2023, any reasonably self-aware rpg company is going to make you sit in the corner and think about what you did just for pitching the idea of a "jungle goblin," but in 2004, you start their entry with "If monkeys were evil and could speak. . ." Yikes.
It's tricky, because there's only like five sentences in this book that need to be nuked, but they're all in sections that make those sentences inevitable. Just like how there was no way a sanity system was ever going to be sensitive about mental illness, the very idea of doing climate-based variants for the standard fantasy races was doomed from the start. At it's best, it's "gnomes domesticating wooly mammoths," and at its worst, it's recapitulating colonialist stereotypes (though, at least, they had the good sense to exclude humans from this system), but even the middle of the curve is too boring to be worthwhile. Oh, desert dwarves are "acknowledged masters of locating water and digging wells," um, . . . I guess that tracks.
What the section really needed was to dig deeper and really get into the guts of using fantasy creatures as a tool for world-building. But, of course, if they'd done that, there wouldn't have been enough room for alternate level adjustment rules, or the bloodlines system, or racial paragon classes. Call it a weakness of the format.
There's much more I could talk about (like the way Complex Skill Checks and Incantations presaged some of 4th edition's best ideas), but I'm not sure my random observations would build to anything. This was like reading a dozen mini-supplements in one, and most of them were dry and technical. There are plenty of inspired ideas, but if you, for example, followed the logic of the spelltouched feats to their ultimate conclusion, you'd have to start asking tough questions like, why do "standard D&D feats rarely give your character overtly magic powers?" You could get a lot of use out of this book's rules tweaks, but you'd have an all-around better time if you simply played a game that took its ideas and ran with them.
Ukss Contribution: The system for magical bloodlines has the basic 3.5 flaw of forcing players to give up levels for things that are not worth an entire level, but if you put the rules on the back burner for a second, some of the setting implications are interesting. Like, the Doppelganger bloodline. They don't get any kind of shapechanging ability until 8th level, and even then it's just alter self once per day, but at 4th level, they get a +2 bonus to Disguise checks. That strikes me as interesting.
There are two possible interpretations of this ability. Either they have some extremely minor innate shapeshifting, below even the resolution of a low-level spell, that gives them a bonus to changing their appearance, or they have a psychological affinity for the Disguise skill, bolstering a learned ability.
I find the latter interpretation especially fascinating, because if you think about it, Doppelgangers are the people on this earth with the least use for a mundane Disguise skill. Doppelganger scions have a bonus to an ability their ancestors never need to use. And I think about the experience of someone who maybe doesn't know they're descended from a shapeshifter. They look in the mirror and they feel something missing, that they can't quite explain, a discomfort with their fixed form. There's nothing they can do about it, but then they discover makeup and prosthetics, and they learn to change their appearance that way. Something clicks into place, and they learn to use the tools available to them with special proficiency. A drive, inherited from ancestors they never knew, gets channeled and repurposed into something they could never have anticipated. I like that. It has a rare combination of mysticism and ecological reasoning.