IMO, in practical terms, 3 key things should imapct instance choice:
- Basic instance rules (including things like community creation policy, nsfw allowed, etc)
- Federation policy
- Instance infrastructure (hardware & how it's managed)
Content specialization really shouldn't matter IMO, because as long as the federation policy is OK for you, then you can participate in any communities, regardless of what instance they are on. In other words, even if you're super interested in french cinema, there is no need to centralize all users interested in this topic on a single french cinema instance. Thanks to federation, users from all instances (accounting for federation policy) should be able to become fully fledged participants in any french cinema communities.
Of the points I listed above, #1 and #2 are easier to include in an instance introduction, I'm not sure how to properly and reliably reflect #3 in any kind of overview. At the end of the day, I think most users tend to figure out their long-term home instance a while after they first join Lemmy, and quite often, it's not their original instance, so maybe it's not that important to emphasize the initial instance choice too much?
I think community discovery can (and should) be improved for sure!
Currently it's true that you can use topic-centered instances for this, I do this myself as well, but I do think it has quite significant downsides in terms of creating pockets of centralization. For example, if you're a user who is ONLY interested in french cinema (or any specific topic) on Lemmy, and all of the related communities and other invested users are on a single instance, then for you, the experience is absolutely no different from any centralized platform - the french cinema instance admins have 100% control over your Lemmy experience.