Is it only about semi and fully automatic mecanism ?
The M6 isn't manufactured anymore, so any M6 you would be buying will be used. That might be a large part of the price difference. The M6 successor, the MP, costs exactly as much ($4995) at B&H as the M7.
There's a decent article of the M7 (with comparisons to the M6) over at Luminous Landscape.
Please do remember that with Leicas, just like all other cameras, the camera is 20% of the tool, the lens in the other 80%.