I see where you're coming from and understand completely, which is why I was so surprised this pack didn't hold up as well as I thought it would. I know games don't age well, and I never thought this pack had outstanding visuals, but now with more experience with the engine and a better understanding of the do's and don't's of level design (in both the Unreal engine and in general), from personal experience in the design process AND from a player's perspective after playing lots of games, I think I can safely say that I was not exaggerating.
I DID enjoy it still, but not as much as I had before, as now I was much more able to see all the faults I didn't pick up on before, stuff I wasn't really able to get a good sense of since I didn't have the mapping experience and design experience to judge them by.
Look at other old map packs like The Elder. That was a fairly good pack for its time and scope, and the reviews still are fairly accurate. But that pack also had much better story, construction, etc. Even if it wasn't great, it was definitely better than One Day. Giving The Elder a lower score than One Day doesn't really make sense, as far as I'm concerned. And believe me, I see all the flaws in Unreal itself. That is no perfect 10, not by any standards.
Still very very good, but not perfect as I used to think. And that's not just because it's gotten old.
Look at Blood, for example, if you've ever played that game. Fantastic level design. For its time, it looked great, and it STILL looks great in a lot of ways despite being an ancient low-poly sprite-based game with limited 3D environments (like Doom). Despite its age, its maps still play well and have great balance and innovation, and the design of its levels is believable and realistic for the most part. When you play through the haunted hotel, for example, it really looks and feels like what a haunted hotel should look and feel like despite being low-poly.
One Day doesn't get points off for being old or low-poly or underdetailed. I don't think the build holds up because I can simply see that, aesthetically, it just really isn't that great. The best parts, visually, were the town that you couldn't directly access and the castle in the final playable map. But even those aren't particularly fantastically built, and with more experience from mapping and playing games, I can get a better sense of what's wrong about them.
My gameplay assessment definitely doesn't have to do with age, because as far as I'm concerned, gameplay doesn't age if it's done right. Unreal's gameplay is still as fun as it was when I first played it. Although I'm better at the game now and need a difficulty-boosting mutator like Limbo to still feel challenged, the core gameplay is the same, and it's still good. I've done a lot of gameplay design; I've spent years and years editing and testing and experimenting with gameplay concepts in EXU, all while playing lots of different games in the mean time and seeing what's good and what's bad. I'm not like a gameplay guru or anything, but I definitely have a much stronger grasp of what makes good gameplay now than I did when I first played One Day, and now I can see that One Day's gameplay has NOT really held up that well. It's not bad, and it's still fun enough to run through, but not more than once in a few years or so. There's virtually no replay value in it aside from nostalgia, and there ARE a number of gameplay design choices that are sub-optimal.
Anyway, I guess our opinions will continue to differ, but don't assume that my appraisal is lower simply because I let the game age. I take that into account when I go back and play old games; sometimes they age poorly, and sometimes they only age poorly in one aspect, like graphics, which aren't as important, like with Deus Ex. The game looks pretty bad now, but the gameplay is still totally awesome and the story is one of the best you can find in a video game, still.
That game has aged well despite the visuals. Some games, like C&C Red Alert 1, aged poorly on the gameplay side, but not so much on the visuals. Replaying that game, I can see how poorly balanced it is and how it's really not that much fun anymore.