Lower prices: Intel and AMD control most of the x86 market, giving manufacturers little realistic choice when it comes to processors (and Intel holds most of the consumer mindshare). In contrast, the company behind ARM doesn't actually make chips itself. Instead, it creates reference designs that are licensed to manufacturers. Thus, there are many companies producing ARM chips, such as Texas Instruments and Nvidia, and all are fighting for customers. Chip prices are extremely competitive, and those savings can be passed on to consumers, especially in a marketplace where ARM devices are competing directly with more expensive Intel and AMD offerings.
Fewer viruses: ARM and x86 are completely different computing architectures. It's not yet clear whether Microsoft will provide a compatibility layer that lets users run older x86 software on the ARM version of Windows, but it's a technically difficult task. Instead, users may need to have software compatible with the ARM versions of Windows. The result? ARM users will be guarded against malware targeting x86 Windows. That's not to say cybercriminals won't start creating ARM-based malware. But an ARM computer will be immune to the hundreds of thousands of viruses that currently target Windows.
Poor performance: Low power consumption comes at a price. An Intel x86 chip will be a lot faster in real-life tasks than an ARM chip running at the same clock speed. Although ARM chips get faster on each release -- the top-of-the-line ARM Cortex A-15 design can run up to 2.5GHz across multiple cores -- there simply isn't the need for speed that drives Intel and AMD designs. On the other hand, there are many who argue computers are plenty fast enough nowadays for everyday tasks. It's only gamers who might miss the out-and-out performance.
Gaming won't be great: ARM chips are based on a System-on-a-Chip (SoC) design. They aren't pure CPU silicon, like most Intel or AMD x86 chips. Often ARM processors integrate 3D graphics -- Nvidia's Tegra 2 chip incorporates the GeForce GPU, for example -- but these are many generations behind the latest discrete graphics cards for PCs. In other words, 3D gaming is possible but don't expect LA Noire levels of realism just yet. However, all the signs are that mobile gaming prefers a simpler approach compared to desktop counterparts -- consider Angry Birds, for example.
Not 64-bit: All ARM chips are currently 32-bit and ARM has yet to release a 64-bit design. In theory this limits ARM computers to just 4GB of RAM, but in up-coming ARM chips this limitation will be bypassed by including a 40-bit memory controller that allows ARM computers to access up to a terabyte of memory --enough for the next 5 to 10 years, at least.