Improvement in memory management starts it off. That sees less information sent out to the paging file for virtual memory keeping more in the active ram.
It doesn't necessarily the 64bit is faster then the 32bit overall however. The first thing everyone looks is not being limited to the 3.5gb of available ram due to the 4gb maximum for the 32bit kernel.
As far as the endless variety of softwares retail or otherwise you'd be surprized at what you can actually get running on the 64bit 7 while maybe not so on the 64bit edtitions of XP and Vista alike while having been written even for the 32bit XP! With anything new you also have to expect some things are going to be lost as well.
With each particular program even with some 32bit device drivers
it's best expressed as what they call "the luck of the draw". Sometimes you can get something 32bit to run while the next time it won't! But the same goes for a 32bit program to a newer 32bit app where the next version simply won't run an older program written for the previous version.