The thing that I think drives the Decidueye discussion is the fact that even if its overall viability isn't tied to it doing one particular role. Say what you want about things like Primarina, some of the Tapus, Alolan Marowak, or even some UB's like Pheromosa, but we already have an overall good idea about what a lot of these mons do. Decidueye's traits don't make it a mon that would manage A or even B tier viability in any one role perhaps, but it could have such a variety of equally viable options that the flexibility within a niche could make him a considerable option. At the risk of a perhaps misplaced comparison, it reminds me a bit of what kind of things went on with Garchomp during Gen 6 (I DO NOT claim that Decidueye is anywhere near that level of "OU staple" no matter what we find with this right now), where different sets rose and fell in viability with the meta to keep him where he was: Tank Chomp, Scarfchomp, SD + Lum sets, things like that kept seeing their little time in the limelight (some longer than others for sure) depending on which was best suited to the meta. If Decidueye turns out to have 5 sets in the SuMo meta, with at least one maintaining something like a B- viability regardless of where the meta development goes, I personally think that's a well-designed mon in that he can adapt to trends without outright suffering for it (even if he doesn't go particularly high), compared to things like Kabutops and Kingdra in Gen 6, whose viability could jump around depending on how viable the rain playstyle was and were over all linear in their options.
No one is claiming Decidueye is going to shake the game up, but he's a mon that is showing surprising potential on a workhorse level with a lot of ways to play him that all have comparable merit. It definitely lends itself well to discussion of the mon.
Another facotr I think is that the metagame is still so new, which not only contributes to discussion (partly on new toy syndrome of course) for Decidueye, but the meta hasn't developed too many trends yet (unless you count "Pheromosa breaks you in half" as a trend), so it's harder to zero in on "this is what a team needs to prepare for, what does Decidueye offer to my cores?". Say Sand were to pick up once all the quickbans and early suspects settle. Decidueye performs well against teams like that, so people would home in on discussing what traits make him strong against Sand and what cores benefit from those traits of his. There's not as much of a focused meta yet, so everyone's trying everything to see what can work, and Decidueye just so happens to have a lot of "things" to try even though he'll probably end up with a relatively concise kit of niches and sets in the OU meta once we have a more defined OU meta.
