Good news: An iconic cultural institution, like a science museum, a public library, or an art gallery, takes "unique" to a whole new level, making your event a once-in-a-lifetime experience guaranteed to impress. Plus, rooms filled with masterpieces and grand architecture have built-in character, meaning little decorating will be necessary. Public venues are centrally located and easily accessible, and the exhibits will provide a great icebreaker for mingling guests.
Bad news: Rental fees for a four-hour reception run anywhere from $5,000 to $20,000. You'll need to pay for security and liability insurance; hiring a professional lighting designer and sound engineer will be a solid investment, too (older public spaces often have funky acoustics and dark, cavernous atriums). Your vendors will not be allowed access to the space until it closes to the public, meaning they may need more manpower (read: money) to do the job quickly. A long list of strict regulations will likely be in effect regarding decorations, required vendors, accessible areas, and possibly even flash photography or liquids that can stain, like red wine.