For me the important things are:
* lots of pockets for nappies, wipes, creams etc plus one to fit my wallet
* change pad because I don't like to use ones in the shops, so I put mine on top
* external pocket for my mobile so I don't have to dig through the bag when it rings
* d ring or hook of some sort so my keys don't end up in the bottom buried
* cooler section for drinks when they get bigger (or for bottles if you FF)
* a strap that will fit properly onto the pram
* and of course something that looks great