Much is arguable. Speaking mostly of obsd.. tighter integration of kernel + userland? Generally smaller code base, generally smaller base systems? Smaller attack footprint? Arguably more secure? Arguably simpler configuration? Better pedigree? Closer adherence to UNIX ideology? No systemd? Better documentation?
Back in the day, BSDs were regarded as being more stable and having a better network stack, though I don't think this is true anymore. Only thing I miss as a desktop/server BSD user is occasional application support, and maybe the improved performance of linux. Obsd is definitely slower but I could never go back.