What I've always thought was cool about jpop games was that they always tried to do something cool and inventive. WCS had the goalie blocking shots, the player controlled kickout for the "assist", and a neat magnetic lock. TOM had that amazing vertical ball lock on the trunk, and CV had the neon light, ringmaster, and more great use of magnets, plus the DMD reposition and synchronization (the effect with the highwire lock is really well done for example). They're all things that made you stop and notice the first time you saw them, which is why these games did well on route at the time (at least WCS and TOM), and continue to do well with new players.
I'd also lump software and design together-they're both parts of the same equation. I also have a feeling that some teams are more collaborative than others, with the "designer" kind of directing the project and working on all aspects of the game, including input on the software. There's no way you can design a good playfield in a vacuum, having no thought as to why you'd shoot for something.
