The key to LocoRoco's success is two things: first, the fact that unlike Yoshi's, for example, its unusual controls don't rob it of the fluency of a 2D platform game, instead allowing for a range of subtle flicks and angled jumps that can be every bit as pixel-perfect as any of Mario's or Sonic's.
Subtly different then, but with its roots in familiar places - and like the traditional platform games to which it superficially eschews comparison, the second key point is that it owes a lot of its charm to likable characters and ideas, and a range of level design elements that squeeze your face into a grin with the alarming regularity of a doddering grandparent.
There's a range of helper animals that spit you into hard-to-reach areas, pinball machine bumpers and flippers, smiling bounce-pads and downward steps that sound little tunes as you cross them - and occasionally offer up a hidden red plant when you pause to play a favoured note. The music is a particularly enjoyable aspect - unlike a lot of PSP games which you can play out happily in silence, LocoRoco's memorable little tunes will have you singing along in gibberish and scouring
Yes-Asia in search of a soundtrack (nothing yet, sadly). The LocoRoco even sing along, breaking into a chorus as they are split apart.
Lovely enough on first impression, there are smiles hidden within smiles. It's always amusing to watch the little LocoRoco dropping into a bottle on the end-of-level screen, which holds up to 20 and acts as a sort of percentage marker; but your grin will break into a smile the first time you end the level with square blobs, and a couple of them spill out of the bottle and start dancing around under the level stats.