Cuts down the avatar box on the Synthesis style to basically make the page height smaller. If you look, the postbit, with activity, member group, username and posts stays, but the things beneath it like add rep, the green rep bars, and the options to IM, block, or add as a buddy disappear. Add rep and the green rep bars are now in the space under the post that Quick Reply inhabits.
For those of us with screens big enough to block out the sun, I don't really see much need, but for those who post from work and who, I assume, will be stuck with the horrible old CRT monitors with just enough monitor space to hold the cursor, it means they won't have to scroll as much to read a thread.
EDIT: Hehehe. When you hover over the postbit, the box expands with your IM icons and block/buddy icons.