For me, the weight of the pannier on the bike is the main reason I don't use it in nice weather. It changes the way the bike feels.

The main things I'd like to have in a pannier that my current pannier doesn't have:

1. Better reflective surfaces for increased night-time visibility
2. A good way to mount a Planet Bike Superflash on it and have it facing rearward
3. Some way to hold my laptop in place so it doesn't lay diagonally across the inner space
4. Lots of inside pockets for wallet, iPod, cell phone
5. An outside pocket that will hold a large pump like the Topeak Turbo Morph