The M4000D does at least have polyphonic Aftertouch.
You know I am really liking the idea of a M4000D and M4000Mini stack but that's not until way down the road...who knows maybe by then someone while figure out a way to make a new one with tape and not have it be this huge thing...much like T Rex doing a new Tape Echo...hmm..
These new models certainly have their advantages in terms of reliability, playability, and controllability. You can connect a sustain, a volume, and two expression pedals (one for stepping through the presets) to them, the bigger model allows for individual note articulation with polyphonic aftertouch, you get a huge library of sounds which you can combine in pairs, you can manipulate the pitch, change octaves, and modify the playback behaviour (e.g. the time it takes the tapes to jump back to their beginning), you can control attack and release times, and each sound is instantly available. While almost all of these features are maybe not so special in general, they do enhance the overall playability and can provide the tools for different uses.
It's a speciality, that's for sure. One thing that has always struck me though, is how evocative the Mellotron's sound is. I would even go as far and say that it seems to be one of the main ingredients for many recordings' haunting mystique. And I'm sure that's related to it having always sounded old in a way, like old and dusty shellac pressings featuring orchestral music. Certain King Crimson or Genesis classics simply wouldn't work just as well without a Mellotron. Imagine King Crimson's "Epitaph" being played on a Polymoog, or an ARP Solina String Ensemble, or with just about any random pad sound. I don't think that would work well at all because the latter lack the imaginative depth in sound, as in: "this sounds somehow ancient." And this is not the case because the composition is bad or not universal enough, but rather because the references to old, European music traditions alongside lyrics referring to myths that are all juxtaposed with Rock ingredients seemed to have found its perfect aesthetic equivalent in the sound of the Mellotron. If anything, it shows that a sound can be just as essential as a composition itself.