Double the oscillators, better hardware real-time controls, can do typical analog poly sounds, but lots of extra ground due to samples. It also has a huge set of samples from a very professional sampling company - I'm sure they were paid something.
It has a bigger mod matrix, and two real time touch faders over the Rev2. The Prophet X also has two DCO's (Sine, saw, pulse, supersaw) adds two sample oscillators and a true stereo signal path. It has two oscillators (saw, triangle, saw+tri, and pulse) + sub osc and 16 voices, 24db filter, and a smaller mod matrix. Whether that’s true for others isn’t particularly of concern. I’m very much looking forward to having this hybrid blend of extensive samples and analog filtered warmth in my setup because it fills a gap for me.
Then there’s 200GB of SSD that I won’t be tying up in my laptop, and a decent keybed that’s weighted radically differently than my other current options in addition to everything mentioned about real-time controls. In practice, 16 channels of high-quality self-oscillating resonant filters is pretty CPU intensive if you’re trying to closely emulate the real thing. It’s true of every synthesizer out there, and again the answer is individual. Likewise, the question of whether hardware is worth the premium over software is hardly unique to this instrument. It varies by individual, and for those whom the answer is $4000 and above they’ll make sales until demand drops and we see how flexible their pricing is.
AnotherScott hit on a number of the points that are relevant to me, but clearly there is no definitive answer to what it’s worth.