The USS Mariner (a baseball blog) has as good a short explanation of where blogs trail newspapers and other outlets in terms of what they can provide. The topic in this case is sports, but it holds up for other topics as well:
I’m (obviously) a huge proponent of blog coverage, but there’s no way it fills the gap of a major paper. We don’t get press access. We can’t go talk to Wakamatsu or anyone on the team unless we know them personally. We don’t have the ability to spend eight hours interviewing people about a breaking issue and turning around something insightful for the next day. The research and analysis done here or on Lookout Landing or anywhere is done essentially for free (well, not Lookout Landing, obviously, as they get to bathe in a hot tub of Kos’ money every night). There’s a lot you can’t do as a writer when your budget is zero.
This disparity isn’t as large as it once was — Talking Points Memo alone has shown that “blogs” can break big news stories, but sites that do commentary are reliant on the professional, full time media to dig up the news that they comment on.