1. How far will you be riding on a regular basis and how hilly is the route?
While a cushy upright 3-speed is great for shorter trips, for long and hilly commutes a lighter, more aggressive bicycle with derailleur gearing might be more appropriate. And for those who plan to to ride after dark through remote, rural areas, investing in a good lighting package is essential.
2. How do you plan to dress on the bike?
If you'd like to ride in your everyday clothing, your bicycle will need the appropriate features. Step-through and mixte frames are ideal for those who wear skirts and dresses on a regular basis. Fenders are a must to keep clothing clean. Many find chaincases and dressguards to be useful features as well.
3. What sorts of things will you need to carry with you?
Different bikes are designed to carry different amounts of weight, in different ways. Do you plan to carry only a light briefcase and an occasional bag of groceries? Your carpentry tools? Your children and pets? Bags of fertilizer and cement blocks? Heavy-duty loads require not only the appropriate racks, baskets, and child seat attachments, but also a heavier build and an accommodating geometry.
4. Where will the bike be stored, and will it require being carried up and down stairs?
For bicycles stored outside or in garages and sheds, weight and size are of limited importance: You can simply roll the bike out. On the other hand, storing a bike in your 2nd floor apartment means you will have to haul it up and down those stairs, making weight and size serious considerations. Furthermore, not all bicycles are durable enough to be stored outdoors, so if you plan to go that route the bicycle must be designed to withstand the elements.
5. Do you plan to ride the bike in the winter?
Not all bikes do well in winter conditions. Here is a post on useful features for a winter bike.
The more complicated and delicate a bicycle is, the more maintenance it will require. Those who do not wish to do their own maintenance and do not live near a good bike shop should opt for simpler, lower-maintance bikes and tires with good puncture protection.
7. What is your skill level as a cyclist?
Not all cyclists pick up skills at the same rate; many struggle with balance and handling issues for years, yet still want to ride. Some bicycles feel more stable and easier for beginners to handle than others. This can make all the difference between whether those cyclists are comfortable riding in traffic.
8. What are your aesthetic preferences?
I strongly believe that there is a benefit to getting a bicycle that you love, that makes you smile. While aesthetic considerations should not override utility, owning a bicycle that you find "lovely" is bound to be more enjoyable.
9. What is your budget?
When determining your budget for a transportation bicycle, I suggest focusing on the transportation and not on the bicycle aspect of that phrase. "How much can I afford to spend on a non-motorised vehicle?" is a good question to ask yourself. Then look for something in that price range, and prepare to be flexible. Transportation is important; don't undermine things like safe handling, good lighting and puncture-resistant tires just to save a few bucks.
Hopefully these ideas are helpful, and any additional thoughts are welcome. Transportation cycling has transformed my life over the past three years and I hope it can do the same for others, regardless of what sort of bicycle works best for them in this role.