I think it's kinda neat that one of the most endearing features that most people seem to LOVE about the Hammond was actually a flaw in the design. The key click has become a staple of "the sound", and that was never factored into the design of the organ. There are differing opinions on how it is created, but here's a pretty reasonable explanation.

http://www.organforum.com/forums/showthread.php?14238-The-Source-of-Hammond-Keyclick

It's like a Rhodes - without the key noise, and the variances in attack (spanks), you just can't duplicate the tone exactly. Sometimes, a little dirt is a good thing.