Alan Greenblatt in the San Francisco Chronicle has a good column on the subject. Never mind the peevish-sounding headline (and always remember that columnists and reporters don't write the headlines for their stories, and generally aren't even consulted when the headline is written) and focus instead on the body. An excerpt:

Habitual excuse notes [in which the writer sheepishly announces that he or she has something better to do today and simply can't come out and create] were starting to bug me until I realized that blogs perform much the same functions that personal letters used to, back in the days when the U.S. mail was associated with the agile pony rather than the pokey snail. After all, 98.7 percent of all personal letters ever written begin with the same apology. "I'm sorry it's taken me so long to write, but . . . " . . .

The worst letters were exactly like the most useless blogs, filled with daily trivia and accounts of hobbies or personal comings and goings that only a mother could struggle through.

On the other hand, letters at their best had all the qualities that make many blogs attractive. They provided the correspondent's unfiltered and immediate impressions of the events and artifacts of the day.

For example, the letters of the late actor John Gielgud, published last year, read as if they were selections from a witty, catty blog about film and the London theater. . . .

[L]ike letter writers of old, [blog] creators hope to forge a connection with readers whose attention they have earned by dint of their own insights or prose quality, not through affiliation with some established publication. . . .

The letter analogy is of course quite incomplete (and I'm sure it's not intended to be complete), but it captures an important point, I think.