Basically from something I posted elsewhere...
In terms of the sounds, they're moving from the sounds of the old Fantom S/X (and maybeG?) that the Juno DS was mostly based on to ZEN-Core, which they seem to be largely standardizing on (even the new Go Keys 3/5 use it). I'm not sure that's a step forward, though. I like some of those older Fantom sounds in the DS better than the zencore sounds on the current Fantoms (though the reverse may be true as well, I'm not sure). In fact I've always been disappointed that, while the Fantom-0 has the old XV-5080 sounds, it's missing the sounds from those Fantoms that followed. Of course the Fantom-0 has a bunch of additional types of sounds... the VTW organ engine, all the SuperNATURAL acoustic tones, and the optional model expansions, but none of that seems available on the Juno D. Though the Juno D does have far more sounds than the Juno DS.
Sounds aside, here's what I see as some of the bigger changes from the Juno DS so far (mostly--but not all--for the better):
... effects on up to 8 parts instead of up to 3 parts (though the DS used to let you put all 3 effects on a single sound if you wanted to, this seems to be strictly one part-specific effect per part, which is consistent with other zen-core boards from what I've seen)
... seamless switching, presumably without the effects glitches you could get on the Juno DS's implementation of seamless switching
... nicer display/interface
... favorites are 8 banks of 16 buttons (instead of 10 banks of 10)
... class compliant audio over USB
... better support for external MIDI zones
... the 32 bar pattern sequencer has been replaced by a 64-step step sequencer (they've also added a separate rhythm pattern section, but it appears to be limited to 30 preset patterns)
... max 8 parts at once instead of 16 (presumably a trade-off for the seamless switching... even Fantom-0 switches from 16 to 8 parts if you turn that feature on, though here we don't have the option of turning it off to get 16 parts). The switch to the ZEN-Core sound sets also seems to mean no GM sound set, but since they are largely used to play GM sequences (up to 16 parts), that may not matter much since the board can only play 8 parts anyway.
? I've seen no indication yet as to whether this supports more expansions or more user sample memory than the Juno DS