I've been self-hosting Mastodon for a while, mostly using it to share pictures of birds. Here are my impressions:

Scolding / hall monitor problem

I see this come up pretty often and I think there are quite a few people who need to chill. Asking for content warnings for things that are mainstream-sensitive is reasonable, but those with specific phobias or trauma triggers shouldn't demand that the rest of the world cater to them. It's easy to filter by keyword when there's something specific you don't want to see. Liberal use of hashtags helps with both filtering and discovery so a polite suggestion that someone tag their photo of a #snake as such is a great option here.

A more curated space, such as a Lemmy community is a better fit than Mastodon's public-square format for people using social media specifically to get support for trauma, phobias, and the like.

Discoverability

Discoverability sucks. There's no other way to put it. I see at least three categories of user who aren't well-served by how Mastodon works now.

Individual users have trouble finding their existing social circle. I recently added a friend who has had an account since last November and I had no idea until it came up in a conversation elsewhere.

People following interests probably represent the majority of those using things like Twitter. The current solution is to follow hashtags (itself a recent feature) or use human-curated directories. The web used to rely on human-curated directories as well, and it scales terribly. "Algorithm" may be a dirty word to many Mastodon users, but I think that's a mistake. If not for spam filters using machine learning algorithms, Email would be long-dead as an example. Algorithms run by adtech companies and tuned to be addictive by finding "revealed preferences" (i.e. things we can't look away from rather than things we want to see) are clearly harmful, but optional recommendation/discovery algorithms designed to serve users could be very useful.

People publishing things like my bird pictures want to get their content to people who want to see it. Our needs tend to align pretty well with the previous group.

I should mention Mastodon's lack of text search. I know it's intentional, and I think that's a terrible take for social networking software. Some instances have implemented patches for better search and some compatible software like Akkoma includes in by default. A simpler search using Postgres text indexing is about a dozen lines of code; someone should probably maintain a fork.

It's confusing/intimidating

Some of this may be unavoidable, but the onboarding process could be more friendly. Mastodon is actually trying this in its official mobile apps, but they're doing it by just defaulting everyone to mastodon.social which I find contrary to the goal of building a decentralized network. The joinmastodon.org website presents a list of servers, which may put off newcomers.

I'd like to see both put a randomly selected server from a curated pool of established, mainstream servers front and center for each visitor.

Too serious

This seems like an issue of cultural fit more than a problem to be solved. It's OK that not everybody is a cultural fit, though if Mastodon becomes more mainstream, the culture will change.