The new ATX12VO (the ‘O’ stands for ‘only’) platform completely redesigns PC power as we know it. Intel has removed the 3.3V and 5V rails, so the PSU will only provide 12V power to the motherboard, graphics cards, storage or other internal peripherals.
Meanwhile, the 24-pin ATX connector is being replaced by a new 10-pin connector, and the EPS connector that goes near the CPU socket will only be optional. Even the 5VSB (standby) rail, used by devices such as USB peripherals in order to remain powered, will be replaced with 12VSB (although the USB output will remain at 5V).
Instead, the motherboard will handle all voltage conversions from 12V down to lower voltages. For SATA-powered kit, such as SSDs, hard drives and optical drives, which need a 5V input, the power will now be drawn from the motherboard, which will have a side-mounted SATA power connector near the SATA data ports.
The reason why this new platform is currently only for system builders is because the changes require new PCs to commit to either ATX or ATX12VO with multiple components.
Instead of trying to manage multiple parallel products for ATX or ATX12VO, it’s easier for motherboard companies to commit to big orders for one product, rather than the dozens of models needed for the DIY retail channel. No doubt Intel remembers the BTX debacle, and it won’t be keen to make that same mistake.
Over the last decade, PSUs started to mainly focus on only AC-to-12VDC conversion, then converting 12V to 5V and 3.3V, as the latter are minor rails. ATX12VO basically removes this latter stage from the PSU completely, letting the motherboard handle voltage conversion (which it can already do anyway).
The end result is an improvement in overall platform efficiency, while reducing the net cost. This cost advantage is likely minimal for a DIY PC builder, but it will soon add up for system builders shipping thousands of PCs.
You can study the new ATX12VO specification for yourself here.