With that in mind, people I follow:
1. Are personal friends, colleagues or relevant superstars in my field.
2. Include a biosketch or link to a website that reflects my interests.
3. Have useful, substantive tweets beyond "I need coffee" (though some of this is okay). Their tweets help me in my work or clearly demonstrate expertise.
It REALLY helps if people include a website, their real name, and a profile image. If people aren't willing to configure their profile, they give the impression that they do not take twitter seriously.
What criteria do you have? Add your comments over at the Twitter Handbook.