Yeah, I'm aware of the sustain pedal option for the S.Art.Lite, but then, of course, you lose the sustain pedal functionality, which I use quite a bit. And, yes, modulation control is something that has always been lacking in the Yamaha E-series keyboards -- even the E463/EW410 -- you have some modulation control with the DSP effects and knobs, but not a true vibrato. This S.Art.Lite button -- when used with a non S.Art.Lite voice -- looks to finally change that, though I'm not 100% sure if that modulation is a true vibrato or another type of modulation. In any case, it's nice that it's there -- especially on the E300 series.
As far as a Leslie effect, it is already available on the E463/EW410, and was also included in the E453/EW400. It's one of the 10 DSP effects, and it's speed and depth can be controlled by the live-control knobs. It's been a while since I played with it, and been quite a while since I've even seen an E453 or EW400, but I remember that the effect was too subtle on the E453/EW400, even on its highest depth setting, whereas Yamaha seemed to correct that on the E463/EW410.
I did find another video from Jeremy See on YouTube about this keyboard, and there, he explained that Yamaha made the S.Art.Lite effects controlled by a button, instead of by keyboard velocity, to allow beginners (who might not yet have developed much control over the force used to hit the keys) to more easily take advantage of these effects. Still, a the ability to select whether hard velocity triggers S.Art.Lite would've been nice.
Another thing I saw, which I believe I saw in the other video that I found, is that Yamaha is including a freeze function with this keyboard. Apparently, you can select to freeze the style, transpose, and maybe even the voices when switching between registrations. That is a very nice feature. On the E400 series, I kind of program a freeze function for styles by saving each registration without a style (by simply making sure that the "song" button is selected before saving a registration), which allows me to easily change registration sounds while playing without changing the style at the same time. The freeze function should make that more intuitive.
As for transpose, that makes perfect sense. I always thought that it was strange that, at least on the E400 keyboards, Yamaha saves the transpose setting to a specific registration. Normally, if I play a song in, say, the key of D, the main reason that I'd want to play it in another key is if I'm playing along with a singer or band that sings/plays that song in a different key -- and in that case, I would want all of my sounds/registrations to be transposed, not just one or two of them, and I would not want to go into each registration and reprogram them for a different transpose key. So, transpose freeze should make this possible, as well.
Here's the other video I found...
https://www.youtube.com/watch?v=mrxrWUizI2c