I'd argue that 16-bit mode is conceptually more complicated than modern 64-bit modes, at least for userspace. 64-bit mode has a single flat address space (FS/GS are just opaque offsets into this address space), memory-mapped I/O, and a single system call trap instruction. 16-bit mode has segmentation, I/O ports, multiple trap instructions (and call gates!), address space wraparound...