Dittos to Dale Emery. I'd add:
Legally, the author of any creative expression owns the copyright to that creative expression unless and until they sell it or give it away.
Many blogs, letters-to-the-editor columns in magazines, etc include text somewhere that says that by submitting your comments you agree to give away some or all of your rights.
In the absence of any explicit agreement, I'd guess that a rational court (hopefully that's not a total oxymoron) would say that by submitting their comments to a blog, people are agreeing to an implicit contract to give you a license to post their entries, or why did they submit them?
But making it explicit is probably a good idea. What rights did you want to have? In my humble opinion, it's reasonable for a blog owner to say that he wants a perpetual, worldwide, non-exclusive license. i.e. you can keep their post up forever and make it available anywhere, but they can use the same material elsewhere if they like.
Note there's a difference between "giving up your copyright" and "giving someone a license". You can let others use your work without giving up your rights. It's very much like renting an apartment to someone: You still own the apartment, you're just letting them use it. You have a contract, perhaps written and perhaps oral, that says what they will pay you, how long they can live there, and any other details either party considers necessary to specify (like can they have pets, who mows the lawn, etc.)