if it's strictly a Nexus device, then it has an unlocked bootloader and no bloat, so it's extremely easy to root, add custom roms, overclock, etc. also, OS updates came directly from Google. however, with non-Nexus devices, the bootloaders can be locked, and it's much more difficult to root. also, you have to wait for OS updates to come from the OEM and carrier, because they have to be tweaked so as to run with all the tweaks that have been made to the OS. beyond that, if it's not a pure Nexus device, but a Nexus-type device with some other branding (e.g., Droid Prime), such a device is generally thought to be much more open, not locked down, and more readily available to accept OS updates.
the OG is an example. it is a Nexus-type device, inasmuch as it didn't have a ton of bloatware (no Blur, minimal pre-loaded system apps, etc) and the bootloader was pretty open. but, it wasn't a pure Nexus device, and carried the Droid branding. yet, it was easy to root, rom, and overclock. however, later Droid devices (DX, D2, etc) had locked bootloaders and lots of bloat, and weren't very open for custom development (at least not without a ton of effort).
so the debate here comes down to whether it will be Nexus Prime or Droid Prime. if it's Nexus Prime, then it will be a pure Nexus device, and wide open. but, if it's the Droid Prime, the concern is that it might be locked down, and have lots of bloatware. the other possibility is that, even with the Droid branding, it could still be a Nexus-type device (like the OG) and be a pretty open device that is very rootable and open to custom development.
personally, i'm all for the Droid Prime, as long as they use the Droid branding solely for marketing purposes. but, if they attach the Droid name to it, and lock it down, then BOOOOOOOO!!!!