I have moved up from the Nord Stage 1 so I will list the significant improvements:
16x piano memory storage (2gb vs 128mb). You won't be able to store all pianos in XL size at the same time, but at large size they'll all just about fit.
Sample engine - this didn't exist on the Stage 1 at all. Nord provides a wide range of samples for download, some are great, some not so great! New string ensemble samples are lovely, Mellotron of course popular and a lot of nice tuned percussion and ethnic instruments. The Electro can also use these samples but the advantage the Stage has is that they can be used with all of the synth features such as layering, filters etc. The rest of the synth engine is vastly improved with 6 filter types (2 on the Stage 1), formant waveforms, and 'cluster' waveforms. Better LFO and general modulation implementation. Arpeggiator is new compared to the Stage 1.
Effects are fairly similar except that reverb in now per-layer as opposed to global. The EQ section has been improved to include a highpass and lowpass filter. Highpass is useful in removing boominess at high volume - it can also be used as a shelving filter.
Organ is improved, most noticeable in the chorus/vibrato feature and in the key click implementation. The Leslie is better and the 'close-miking' option in the sound menu is nice. Tonewheels sound less shrill in the upper register. Physical drawbars are more fun to use!
Pipe organs in the organ section are just okay (they lack the 'chuff' at the beginning of the note), the organs in the sample section are probably better.
Seamless program changes, which allows the sustain from previous programs to overlap with the sound from a newly selected program, is a fantastic feature for live performance (new to the Stage 3).
Song Mode which allows you to organise up to 400x groups of five programs is another fantastic feature for live performance (also new on the Stage 3).
Much more space for storing user programs (400) and the Sound Manager software for organising and moving samples, pianos, programs and songs is superbly designed and essential.
You can create your own samples for the Nord Stage 3. The Sample Editor software for doing this is ver well-designed.
Things that are unchanged from the Stage 1 Compact are build quality, size and weight and morphing using expression pedal/mod wheel/ aftertouch. Morphing is really useful in making strings and brass sounds more convincing from a keyboard. Also very useful in synthesised sounds.
The Stage 3 interface of controls has now reached such a level clutter and complexity that any further expansion would not fit on the Compact panel, but frankly though that would become a problem as Nord's inventive but quirky way of changing parameters, sometimes through double taps, shift-presses and other combinations is getting confusing and I sometimes hit the wrong button, jump to a new program and lose changes I had made.
In a Nord Stage 4, it would really only be minor or esoteric things Nord could do to improve and advance this machine beyond increasing the memory for piano and sample sounds. The sample playback engine could certainly merit some expansion and improvement (envelopes, velocity layers). They could introduce a keyboard with polyphonic aftertouch but this would necessitate a much more powerful sound engine in order to be able to make use of this and it would probably be of niche interest while significantly increasing the price. Any further sound programmability would have to be out-sourced to a computer or phone as there is simply no more room for hardware controls and the menu-based interaction that there is on the Stage at the moment is not terribly nice. The restricted options for where to set split points seems a bit silly, even though the permanent LEDs to indicate split points are nice. The Stage 3 is still a reasonably future-proof purchase, even though it is a few years old now.
I originally chose the Nord Electro 6D 73 but immediately swapped it for this as I felt I would miss the synth section (thanks Thomann for making this an easy operation), the expanded effects section, the modulation and pitch bend controls and the additional layering and splitting possibilities.
The Nords are well-known for being expensive, but I would say that they are just about worth it, with consistently high quality sounds, solid build quality and light weight and small size.
I bought the original Nord Stage 12 years ago. I hope to use this new version for the next 12 years.