Weighted action:
Well, it certainly does feel different, and uses more strength to play comfortably, but there's more to it Gary. My experience has shown me that there is a wider range of expression with a weighted controller. Using the same sound source, I can play with much (yes, MUCH) more expression when I use a controller with more weight. It's mostly noticeable in piano/clav type sounds, but then, that's my bread n' butter.
Since using the PA4x, I've noticed a big difference from my old PA900. If you've never cut your teeth on a piano, or Rhodes, it won't make sense to you, but those of us who are used to "digging in" and "spanking" out a groove will understand. If you spank a psr ANYthing ... it won't sound any different, and you'll bust the keys way before their time.

Try this:
Strike a single key as quietly as you cam, then gradually increase the velocity till you get to the loudest. How many steps did it take? My guess is that most mid range arrangers can only give you about 4-5 levels of volume/timbre change - some even less (CASIO). When I play my Korg SV-1, I can tickle it, spank it, or pound on it with a variety of expression that's simply not possible on some other keybeds.

With all that said ... I don't use the SV1 on the job. It's strictly a studio piece, or for when I do a dueling piano routine.