When I recently ported a large project from VC6 to VC2005 I found that it was surprisingly easy.
I found warnings on library functions like strcpy(), scanf(), the safer _s variants are now required. and for loop variables which are declared in the loop (e.g. for (int i = 0; i < 10; i++)) can no longer be used after the end of the loop.
There were a few other problems where 2005 is a bit stricter on the typing, but it's not a big deal.
i also found that remote debugging in vs2005 is very easy then vs6
and vs 2005 provide setting for Unicode and Multibyte support for c++ but in vs6 we have to define it in code
like this anything else differences is there