Honestly, the iMac is about as good as you're going to get with an AIO.
A mini-ITX system in a Lian-Li Q08 is going to be a fair bit bigger than an iMac of course, but at least you're taking away virtually all of the restrictions that you get with the smaller mini-ITX cases. You can buy a normal graphics card, and standard desktop components, which means better value for money and easier upgrades. The iMac is great if you're going the Mac route as well, but anybody who is even remotely thinking of running Windows on it, and has no preference between OS X and Windows should steer well clear.