Doesn't really have to be waterproof. Nor does it have to have a Q-R system. Does need to be relatively easy to carry off the bike for minimum time/distance. Got to be able to fit a couple layers of clothes, as well as the obligatory keys, wallet, phone, bus pass, mp3 player, spare batteries...

Large panniers won't work for me due to heel strike. Saddle bag panniers are attractive, but most that I've seen look like they would be awkward carrying around. If I could find some panniers that rode higher (e.g. the hardware was lower on the bag instead of near the top) then I'd consider that option.