I used to carry a small pannier for:
1) lunch
2) patch-kit and a mini-pump
3) thin rain-trousers
This does not take much room, so the small pannier was nearly empty most of the time.
Occasionally I stop somewhere on my way home from work and needed space for stuff not related to the commute-to-work.

Now I have recently started to take my 1 year-old to the day-care on my way to work, so I have a carrier for her on the rear rack and no space for the pannier. I have been using a backpack to carry the "luggage" but need to install a a basket or a bag on the handlebar instead (I hate biking with a backpack).

My needs in a commuter:
1) Fully rigid frame. No suspension at all, except for the front and rear air-cushion suspension (a.k.a. tires).
2) Good full fenders, preferably aluminum. Front fender has to be larger than usual in both directions and should have a mudflap.
3) A strong rack.
4) Clearance for 2" wide studded tires. I need these but I realize that this is somewhat unusual demand.

My wants in a commuter. These are things I would like to have but can't afford:
1) Front disc-brake.
2) Internally geared hub. (I think single speed is also a very handy commuter, but I do need gears in the winter time).
3) Generator hub.
4) Good front and rear lights.