Problems I've faced on it are: 1) Long spacebar with pressing mechanism in the centre. Pushing from the side (where my thumb is) creates a slight resistance in the centre plastic which then causes a louder clack when it gives way. This is noticeable when working in a silent room, and noticeable on my fingers.
2) Too much key travel on all keys - when I went back to typing on my mac after using my 4000 for 6 months, I couldn't believe how much more relaxed my fingers were after not having to press so far down with the keys.
I think the MS Natural 4000 keyboard is designed for people who aren't aware of their fingers but think they're getting ergonomic benefits just from a split keyboard with a faux-leather palm rest. I tried many of these MS 4000 keyboards and they all had the same design that I consider to be a flaw.