My answer at the time was that a LJ site is more of a public diary, with not so much pressure to deal with the serious political and cultural issues of the day. I personally rant about some of those things from time to time, but that's because it's what's going on in my head, not because I think thousands (or hundreds or even dozens) of people care what I think. Obviously some people do care what I think - or at least find it interesting - since I do get a fair number of comments, but they're on the strangest things sometimes. Take that "Magic Kingdom" shorts post, for example.
LJs also seem to build networks of friends, or maybe just reinforce them, more easily than blogs. You notice comments people leave on your LJ, or on your friends' LJs, and you put them on your friends list, and people do the same to you, and then you wind up meeting someplace, and it's pretty cool. That doesn't happen with blogs so much because they don't have that feature built in.