Well, it's the only mora I own, but here are some reasons I like it, and why others may as well:
*Price (At only $9, what's not to like?)
*Guardless (or nearly so)
*Perfect size for a belt knife
*Nearly full hidden tang (some people just like the idea that the tang goes almost to the very end of the handle)
*Red handle (Some people like it if they ever drop it, others hate it because it is so bright and cheesy looking. I'm in the middle and my solution was to wrap it, looks a little better, but still easy to find with the red ends)
*Obviously the same blade features that most other moras offer.
*light weight
*Handle material is hard and durable, yet still plenty grippy.
*Carbon steel blade.
*Did I mention price?
Of course some of these points also apply for many of the other Mora knives.
I think the price is the biggest selling point for me. You get a lot for the money. Of course if I could afford to, I would also own a nice wood Puukko, but even if I did, I'd still have a 510 lying around as a backup, beater, or loaner knife.
And of course I can't help but post a pic of mine, even though I've done so a hundred times: