I've got a Mac Pro that works very well. Besides adding USB3 and Thunderbolt and updating the video card, I can't imagine where they're going to go from here. I just hope Apple doesn't do anything to further iOS-ify the workhorse of their portfolio but rather reverses this trend.
What they can add further: on-board Flash for OS; Fusion option, newer server chipset (C60x), ESXi support for virtualization; dual PSU, rack-mountable easy way.