The idea behind the reaper is similar to the revenant--an sinister undead character. Matt and I were actually tossing this idea around a while ago because we both like skeletons so much. We thought about what a skeleton PC would be able to do. As with all monsterous races in 4e, the challenge is to make it mechanically feel like the actual monster, but at the same be a viably balanced.
The next challenge with home brew is to make it look and feel different than any other race. Otherwise, you're just making a variation of some similar creature that could have been easily flavored to whatever particularity. The problem of balance also arises. Does your new race completely overshadow the old one? With the kickback to oldschool Aasimar, I had the gleaming problem of the Deva--WotC's officially declared replacement of the past Aasimar. So how could I make the "half-angel" different than the scholarly Deva's? Why, make the Aasimar a soldier of course!
Deva = INT/WIS, astral, memories, blue
Aasimar = STR/WIS/CHA, gold, glory, whispy
I wasn't trying to blatantly rip off Hellboy with the Nereid. My thought process was, "What could be a new 'Merfolk' race that would not be stupid and actually be fun to play?" While brainstorming, a flood of blue magic cards raced through my mind. I decided that magic card designers got it right--an creature that spend his days lurking through sunken ruins would be bound to come across forbidden secrets of the mind, not to mention all the "6th-sensey" glands to which dolphins are already attributed. The goal became to make a psychically flavored fish-man. As I began to photoshop the artwork, I realized that the architecture for the quintessential blue fish-man had already been set out for me. The universe revealed itself to me and the race practically designed itself.
You may think its dumb to rip-off such a contemporary comic book movie like Hellboy. Let me remind you that everything in D&D is a rip-off. Plus I think Hellboy is cooler than most silly old mythology from which modern epic fantasy is derived.
I spent hours trying to differentiate this guy from the minotaur. The bariaur is, of course, a throwback to the 2e planescape goat/centaur. What encounter power would you expect a goat man to have? What encounter power does the minotaur have? Hence, my hours of toiling with how the hell I was to back of from Goring Charge territory. The only answer I could come up with was that bariaurs are more "fey"-ey, thus the would have more exotic, primal power built in. I'm open to suggestions on how to make this guy better.
Let me know what you think. Also, give me suggestions of other races would like to see in the 4e dynamic.