What needs to be considered when building a winter bike? I have been thinking about making the ultimate winter bike.  Here are some of the things that I have been thinking about.  What else would you think about when building a good winter bike.
I am specifically building this bike for Ottawa, Ontario, Canada.  The average January temperature is -10°C (12 °F) but it can get as cold as −30 °C (−22 °F).  I am interested in making a bike that will ride well in both urban and more rural areas.  It should be able to handle snow and ice.
Frame

Road frames generally have less clearance for wide tires, mud guards etc.
Even when the roads are mostly clear in the winter I often encounter snow filled streets and icy patches.  I am leaning towards a mountain frame because I think that it would be better suited to this kind of semi-technical riding.
I have never used drop bars in the winter before I really liked having a long handle bar.  I found that this really helped to balance on slippery roads.  Seems like if you want drop bars it is best to start with a road frame because adding drop bars looks like a pain.
Corrosion -  An aluminum frame will hold up better than a steel frame in the winter.  I think that the salt will do a number on steel, aluminum should hold up better.

Suspension
Last winter I decided that I want to get a new fork this winter without suspension.  In the summer I feel totally comfortable jumping off ledges on my bike.  In the winter I would not want to jump off anything that required suspension because in winter it is too hard to guess what the traction will be like when landing.  I also found that my suspension froze so even if I had wanted it it would not have helped.
Tires

Studs help grip on ice but will not help in snowy weather.
Wide tires help to float the rider on top of the snow.  If you want to have really wide tires you need to get wider rims.  You can find rims up to 44m wide.

Brakes
I love my disk brakes.  They are very reliable and I trust them.  I have not used rim brakes in the winter but I know when they get wet they grip less well.
Since I want no suspension and disk brakes it is tricky to find a fork.
Lights
In Ottawa the sun can set as early as 4:30.  To see and be seen serious lights are required.
Mud Guards
Last winter I did not use mud guards.  I found generally everything was frozen so I did not get too wet.  I think after I remove my suspension I may add some.
Conclusion

Aluminum Mountain Frame
Disk Brakes
No Suspension
Studded tires (wide if going in deep snow)

What else would you think about when building a bike for the winter?  I am hoping to get a used frame and used parts when possible and make the studded tires.
Good for you, to think of riding as a year-round activity.

I would choose an older steel frame (Specialized Rockhopper from 1994?). Steel is very durable, and the older frame is less to be worried about with salt. Put a little FrameSaver on the insides, and don't worry about it. (I'm not an expert on these, just a Grant Petersen fanboy).

You really don't want to get stranded in an Ottawa winter. Add some storage, and carry:


a plentiful toolkit, including a multitool, pump, and spare tube. 
cash for bus &amp; phone
spare batteries for your light
compact warm items, like a balaclava, gloves, and a wool hat


The trick to rim brakes in the wet is to dry them out before you need them. When you anticipate a need to stop (approaching a driveway or intersection, for example), lightly apply the brakes. After one rotation of the wheels you'll have most of your braking power back. After a second rotation, they'll be in great shape.