I think that one of the big concerns when choosing any pet is how it will potentially react to the rest of your family. For example, if you have small children, large or aggressive dogs would probably not be a great idea. Even some tiny, fragile dogs will not work out with small children since they feel vulnerable and will defend themselves if they feel it necessary.
You need to consider how much space you have outdoors for your dog to exercise and play, because it's unlikely you'll always be available to take it out when it needs/wants to.
Money is a big issue as well. Dogs require food, bedding, toys, vet visits, etc. If you can't afford to feed a large dog are take it for rabies shots and annual checkups, it's probably wise not to get one at all.
Your time is also a factor. Dogs usually crave your love and attention. If you're gone all day at work and too tired to take it out for daily walks, your dog will be extremely unhappy. However, if you work from home, or have a job that allows you to spend adequate time at home, then you should have no reason to spend time with your dog.
On a side note, some would consider where you got the dog to be a factor in choosing one. If you want a pure-bred dog, make sure you research where you are getting it. If you want a dog to be a good companion and not a good show dog, then I would suggest your local animal shelter. These dogs are always in need of the love they deserve. But again, research where the dog came from. Some dogs are turned into the shelter because they were abused or even abusive. Both of these could cause psychological damage to the dog, and it may be hard to trust. Puppies are good to start with since they haven't been highly impressed upon yet, but they take extra work to train them the way you want them to act!