> Even if you shutdown or switch to a different computer, your browser will always be on. Exactly how you left it. That means you’ll never lose tabs that remind you of something to do or contain an article you want to read later. You can quickly jump back to work.

I haven't seen that discussed here, but it seems key.

Of course Mighty is not the only way to implement it; if Mighty takes off — or even if it ends up looking like a good idea — watch Chrome natively implement something equivalent.