We covered many of the visible features of the GA-P55-UD6 above during our walk around of the board itself. However, there are some not-so-visible features as well.
GIGABYTE has thrown in a full 2 ounces of copper into the construction of the board as well as adding in their Ultra Durable 3 technology. You will also find some great power saving features under the hood in the form of GIGABYTE's dynamic power savings.
The P55-UD6 has an on board power button (near the RAM slots) as well as an easy to access push button CMOS reset found near the SATA ports. It should have full support for ATI's Crossfire X and NVIDIA multi-card SLI as well.
One factor (like the P45 before it) is that it will not be full x16 across the board. The additional PCI-E slots while x16 physically are only x8 electrically. This means that your setup will run at x8 across the board if you have multiple GPUs installed.
The P55-UD6 will also have GIGABYTE's DualBIOS support.
Although there is a lot stuffed into the P55-UD6, it is not too far out of the normal realm for a modern mainboard. It is a shame that the SATA 6G support has to be pulled, but perhaps we will see it on future revisions.