Why is choosing your domain name so important?

I mean to say most people interpret domain names in common sense and they don't follow what you lead them to. In order to prove that I am right, please look at the following unfortunate names:

1) Who Represents?, a database for agencies to the rich and famous:
http://www.whorepresents.com

2) Experts Exchange, a knowledge base where programmers can exchange advice and views:
http://www.expertsexchange.com

3) Looking for a pen? Look no further than Pen Island:
http://www.penisland.net

4) Need a therapist?
http://www.therapistfinder.com

5) Mole Station Native Nursery, based in New South Wales:
http://www.molestationnursery.com

6) New to Milan and you need electric light? Why not sign up on-line with Power-Gen?
http://www.powergenitalia.com