There is something inherently attractive about putting every piece of your PC into the same enclosure, especially if you already favor the minimalist approach. But all-in-ones cut two ways. You can have simplicity, sure. But the tradeoff is accessibility. You can still upgrade a couple of components, like hard drives and memory. However, there's really no way to drop in a new CPU or graphics processor. The optical drive is removable, but don't expect an easy upgrade to Blu-ray. All-in-one PCs employ slim optical drives. This makes upgrades more complicated because you need to match the outer bezel.
While these are acceptable tradeoffs for many people, few enthusiasts tolerate poor graphics performance. Simply put, gaming on a cheap all-in-one is almost necessarily bad. There is limited room in a fully-integrated chassis, and that means a tight cap on thermal and mechanical specifications. The most you can hope for, really, is a chipset-based graphics processor. That means that games like Call of Duty, mainstream though they may be, are assuredly too taxing for an all-in-one. World of Warcraft: Cataclysm is fine, but that's where we draw the line.
Alright, so you give up gaming. But Gateway's ZX4931 ($599) and HP's TouchSmart 310 ($699) are touchscreen all-in-ones. Touchscreens provide the additional benefit of a dedicated software interface. That's what makes these PCs different. They are intended to be used in the living room, kitchen, or wherever your family congregates. HP fully embraces this experience with a much more robust touchscreen interface.
Gateway's touchscreen Web browser has a more intuitive design (tabbing is better) that we'd like to see HP mimic, but HP provides so many more "widgets" that the options seem almost limitless. Unfortunately, HP still fails to provide everything we would like to see. Our ideal all-in-one should be able to double as an HTPC. That means we need HDMI connectivity, too.
You can expect to pay a roughly $100 premium for the addition of touchscreen functionality. That's not too high of a price if you're looking to break from your keyboard and mouse, even if both components are still included with both all-in-one PCs.