I love the scoring for each area's day/night cycle. The way the meandering keyboard part slides into a sweet jazzy lick as dawn is about to break feels so good! So far there are only a few direct rearrangements: Heartwood's music of DPPt's Eterna Forest, for instance. But, during the day time is where the unique "Route" scoring happens for each area from what I can tell; they'll slip into licks and verses that are lifted from DPPt's OST and then move back into either wholly original or highly interpreted tracks that are still as jazzy-bouncy-energetic as I've come to love. Go Ichinose, Pokemon music veteran, is a lead composer on Legends: Arceus, as is - I'm so delighted to say - Hitomi Sato, whose breakout game for composition was in fact Diamond/Pearl. The original OST held close to my heart, their absence from the D/P remakes were a contributing factor to my avoidance for those games (as was a recent 200+ hour in DS's Platinum), and I am just so thrilled to see them back for this title. I pretty much bought the game without doing any investigation solely since it was taking place in Sinnoh which is a fave region so... I'm just happy lol.
The battle system moving to FFX-style turn-based is really interesting and was certainly a move to accommodate the dynamic nature of encounters since sometimes more than one Pokemon will join in on the aggression. It also contributes to an active feel instead of the stop-time that occurs in the microcosm of "the turn" in older titles, as does the minor feature of being able to run my trainer around during battle. This change to turns has a number of ramifications, such as Priority moves' effects being changed to reduced delay instead of leading a turn. A lot of moves had to be adjusted to fit into the new system, and I'm so glad that they're finding the strength to let go of the legacy mechanics. I love all the handheld entries to pieces and most everything about them - and those games will always be waiting for me whenever I want what they have to offer. But this new direction and everything with it just has me screaming "Yes yes yes" the whole way.
Also I love how "Original Partner" is used instead of "Original Trainer" for Pokemon: it is very much aligned with the game's setting and theme. The setting is the perfect reason not to have to design the game around all the global trade/battle/competition aspects and what they have instead is the most refreshing, exciting Pokemon release since I don't know when.
And, field exploration in general feels wonderful. Sneaking through tall grass, catching without battling, baiting Pokemon to turn around, getting swarmed and running for my life- it all plays like the Pokemon game my friends and I used to dream they'd make when games were moving into the third dimension. We get to aim our throws, and Pokeballs have different weight affecting their trajectory and application - throwing Wing balls at things in the sky is pretty much my favorite thing. Rediscovering landmarks such as Mossy Rock and Solaceon Ruins and the three Lakes also warms my soul.
There are things that are missing too, such as Pokemon abilities and held items. My gut says they were likely dropped due to the staggering amount of rebalancing required for the new system, but it's also very possible they weren't concerned to begin with: Battling is as prevalent as ever, but the competitive aspect which those systems best served is not a feature of the title. Breeding is also gone, but that whole concept was kind of a humans-controlling-nature system that wouldn't be fitting for the setting either. These are fun mechanics that belong in the other Pokemon games, but I can't say I've been missing them here. But, I also wouldn't be upset if they were added into future "Legends:" titles, if that is the direction that this is going.
I was pretty bummed when I learned GameFreak wasn't involved in the D/P remakes since I was fond enough of their work on Alpha Sapphire and eager to see similar treatment from them for D/P. In retrospect, however, this Legends: Arceus is the best gift they could ever have given me, and it accomplishes so so much more for Sinnoh and for Pokemon as a series.
... also I found a shiny Wurmple in the wild last night and so I'm still up in cloud nine.