I'm surprised no one has mentioned instant-return mirrors. Not all Hasselblads have them; the SL66 does. I always found finder blackout disruptive, especially when shooting events.
When I shot weddings with my SL66 (in the seventies), the combination of instant-return mirror and the unrivaled close-focusing capabilities were great. I also find the SL66 film magazines a lot easier/faster to load, although I'm sure I could get used to either.
Today I shoot for fun, and I find the SL66 a fantastic camera, either handheld or on a tripod. (although, if I could somehow rationalize the purchase, I'd probably go for a Rollei Hy6 instead. )