I think a lot of this discussion is based around the current list's owners' nostalgia for their youth. "I want to relive the glory years of prog rock when I couldn't afford one of these things!" Now we have the money to indulge ourselves in the same way that a 50-year-old goes out and buys an old Jaguar. I personally would be bored musically with 3-violins, brass, choir and cello (which were the most popular sounds way back when. To say that the "Orchestra" sound is made up of old components, and then blow it off for that reason is completely missing the point. Les did something wonderful when he put it together. To be fair, I love playing "Watcher of the Skies" as much as the next old fart.
The reason the old library of sounds is so dodgy is that they were recorded badly by people who weren't adept musicians (the people recording, not the people playing, necessarily). That brass sound is ridiculous if you break it down into its 3 discreet components -- which is how it was recorded. That's its downfall (but also its charm). Lots of horn bands use trumpet, trombone and tenor sax. Do they sound like the MkII brass? Of course not. If Les and the guys had recorded these three instruments playing together at the same time, the results would have been far different (and probably a lot more in tune!).
My reasons for recording new sounds was primarily to make some new ones available (bass clarinet, bari sax, bass flute, etc) but also to fix up some of the bad ones that I really wanted to use but couldn't (French horn, oboe, clarinet). The reason many of these new ones don't "sound like a mellotron" is because of our list members' expectations still being mired in the past.
What's the median age of the members of the list? I'll bet it's mid-50s. Of course there's a natural urge to hang onto our youth. I'd like to give a few M4000s to some good, creative young bands and see what they could come up with since they're (hopefully) not mired in the '70s. I think we'd all be surprised with what they'd come up with. Would we think it sounds like a mellotron? Perhaps yes, perhaps no -- but they would. Actually, that's what Streetly is doing and there have been some excellent results.
Is anyone here aware of the fact that the precursor to the mellotron was the pipe organ? Take a look at what many of the ranks on them are called: oboe, clarinet, flute, trumpet, etc. In trying to make an instrument that could imitate these instruments, organ makers came up with a wholly new instrument that sort of sounded like them. I've heard mellotrons described as tape transport organs, and that is what they are. I still maintain that any instrument recording put on an 8-second piece of tape and mounted on a mellotron will sort of sound like that instrument, but in going through the instrument, it will become something different. Vive la difference! Those who say the new voices don't sound like a mellotron haven't played them.
Rick