It could be a few pounds lighter than your old Diamondback. I've no idea if your old bike is double-butted CroMo, straight gauge hi-ten, or something in between. The older mountain bikes we have in my family probably run 27 - 28 lb with decent CroMo frames. I would make a wild guess that the BD bike would be around 25 lb. Losing a few pounds won't make a huge difference, although it's nice when you're climbing hills.
Shifting might be improved, although the older bikes we have (3x7 with various Shimano components) still index pretty well with occasional tinkering, new cables, or whatever.
The gearing should end up being higher than your present bike, so if you find yourself spinning out in high gear now, then the new bike will be better. The low end for the newer bike will probably be higher than you're used to.
I've been pretty happy with my BD stuff, but no experience with this particular bike.