You can choose an instance that gives you like-minded people and an intentional community (like feddit.de for a German instance, or programming.dev for all things development and programming related, or ani.social for anime communities, or beehaw.org for a more vetted signup and member approach for a more social and healthy userbase)
Lemmy is federated meaning despite this separation into instances users can read and participate in communities and posts of other instances
Instances can choose to not federate or to block other instances according to their choices (another reason to choose your instance according to your intentions and expectations or usage pattern)
You can link posts, add text to the post, and edit post titles after posting
Those are probably the most obvious and usage facing differences. Additionally:
Lemmy is a platform of free and open source software, open to customizations and collaboration
Lemmy instances are run by groups and individuals, it’s open to people and groups joining with their own instances
As such, both in software source and platform, Lemmy is a community project whereas Reddit is a private company (soon a public company owned by shareholders)
Lemmy has an open API allowing for custom client, bot, and other integrations
Lemmy uses the open ActivityPub protocol, so it can interact with many other platforms like Mastodon, KBin, etc
In many other ways, it is similar to Reddit. Like having upvotes and downvotes. Lemmy is still young, so it will improve in terms of functionality and annoyances.
Those are probably the most obvious and usage facing differences. Additionally:
In many other ways, it is similar to Reddit. Like having upvotes and downvotes. Lemmy is still young, so it will improve in terms of functionality and annoyances.