Okay, for starters, I haven't had any experience with any of the classics like the d555, or even the d-34.
The D-121 sounds pretty darn good. Compared to to the D-e301 discman (10 sec anti-skip) or the Panasonic SL-S200 (again, anti-skip) the 121 seems to have a more expansive soundstage. Stuff just sounds clearer. Also, bass seems to extend far deeper, while on the other two, the bass seems kind of muffled and boomy.
Only takes 2 AA batteries, which actually matters to me. and the industrial design is pretty neat.