Cue wailing and gnashing of teeth. Woe is us, because the evil corporate types at Six Apart will leave nothing standing: they will indiscrimately delete Journals and communities, they will make everything paid, they will do away with development, they will sell off the hardware LJ is running on! Woe is us!
I don't know why it is that people fear this change. It's by no means said that all these bad things will happen, or that LJ's unique structure will cease to exist overnight. LJ, TP and MT are complementary, and I see nothing wrong with a company wanting to extend their product portfolio.
I think it will be a good thing for the users of LJ, overall. I expect support to become much more professional, as the whole support structure will be folded into 6A's existing support framework. Yes, this will be the end of the quaint community support system, but I don't think that will be a bad thing. Volunteer support is like a dog-sled, while professional support is like a car. A dog sled has its charms, but if I want to get from A to B, I'll take the car.
I also think there will be much more focussed development on LJ, as 6A will want to ensure it interoperates well with TP and MT. This is not a bad thing: the two systems are largely separate worlds. More interoperability means a larger community as the two systems move closer together.
And if everything else fails, we could always set up our own LJ-server somewhere. With about 25 users who would otherwise pay for a year's worth of paid account, you're covered with respect to colocation costs! We'd have our own little LJ universe for our own Journals, while maintaining a free account on LJ for stuff like communities and so on.
A wild plan, and I don't think it will prove to be necessary. Don't get your underwear in a wrinkle just yet.
EDIT: It's official now. Brad's announcement over at news talks about the how and why, and provides some links to more information. In short: not much is going to change from the point of view of the users. You can all go back to sleep now. 6A isn't evil.
I'm really excited about this, I expect LJ to become a lot better design- and usability-wise -- traditionally a weak point of LJ.