MS specifically chose to make some significant security changes in 64 that they could not make in 32 (For backwards compatibility). So, the downside is no 32 bit layer and required kernel driver signing...the upside is no 32 bit layer and required kernel driver signing. That means no malware or spyware can insert itself into the kernel b/c it won't be signed by microsoft. It also means that a lot of open source projects won't work and that hardware manufactures are likely to not port drivers for consumer (and older) hardware...at least until there is a big demand.

I'm just looking forward to Leopard