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“What’s so great about Hexbear? Why do people keep talking about Hexbear? What’s so special about this instance?”
(hexbear.net)
Maybe it is more than an instance. Maybe it is an idea, a world-historical vanguard, light itself. 
actual attempt to answer (cringe)
We’ve been around for three years on a lemmy fork before federation. The lack of federation for years as well as the place we splintered off from (/r/chapotraphouse) already having a unique microculture within Reddit has given this instance, in my opinion, a distinct culture compared to general-purpose and even other hobbyist instances.
Regarding specifics, Hexbear is a vehemently queer-friendly/intersectional anti-sectarian socialist instance. Check us out 
All of us. All at once. Unannounced. We just kick the door open and keep piling in to the studio until we all smother.
The proverbial Big Richard Energy other instances clearly lack.