mainly, women tend to be colder than men, so we usully require a little more insulation. most "unisex" gear tends to be pretty functional for women, but there are a few items where i have found it is nice to have a women's model. pants and shirts definitely. sometimes women's packs are very nice - i have a huge heavy-duty Gregory women's pack that i use when i have to carry a LOT of gear & food (family trips). it has pretty much the nicest fit i've ever had in a pack, too bad it weighs over 6 pounds...
but for the most part, if you can get unisex gear at a good enough price then you don't need to pay full-price for women-specific gear.