I don't really know what attracts me. I think colour (I love non-colours, mostly black and grey), but also texture - I like my bags either boxy/stiff and entirely smooth (i.e. McQueens Novak, Hermes Box Calf) OR completely pebbled and very distressed/textured (like most Fendi Spies, Chloe paddy etc.). I don't like 'in-betweens' (just soft leather without texture). So, I'd say both colour and texture do it for me.
Shape!!! I am very picky about my bags - I like very structured bags, sort of like the Louboutin on the PurseBlog right now Since I hate slouchy bags and hobos, that rules out like half of all bags right there!!
Next is color - I loooove bright colors like pink blue and green, but a sleek sexy black, white or grey bag will make me stop in my tracks.
I recently looked at all my bags together and the ones I use all of the time have something interesting going on with the handles, they are either braided, twisted, or something else funky. The ones I never use are the ones with just plain jane handles. So I guess the handle either makes or breaks the bag for me. After that, it would be material, shape, and then color.
I think whether the shape calls out to me, and also looks functional. Each bag I buy has a special something that appeals to me... the hardware, the pockets, the space, the colour, the straps, the way it looks when you put it one. That's why I need so many bags!