Many things, but some of the important ones are:

It was really and truly developed by a mix of pros and serious users meeting both privately and within BearShare Labs which was available to all in the old BearShare Forums.

It was always designed to be functional and screw the pretty stuff... Hence there was never any 'skinning' nonsense.

It was and still is the only client that actually respected the intelligence of the user.
The GUI is clean and simple with as much info as you wished to see.

It is 'friendly' within Gnutella Network.

...er, and there was Vinnie !

Aaron can give a very succinct run down on the technical aspects that set BearShare apart from the others.

Don't panic !

It is simply an affectionate nickname used in the Texas insurance business.