Increased character limit is Mastodon's least important feature, albeit a nice perk
Mastodon's primary strength are the decentralization (resilience to failures, government censorship, distributed nature of the moderation and ability to own your own megaphone) and the power dynamic between the product and the users. It is free and open source, and does not seek to exploit the user through ads or surveillance; it is, rather, made by the users for the users.