I expect 10.7 will run on all Macs with 64-bit Intel processors. That would mean that early Intel-based Macs with Core Solo or Core Duo (as opposed to Core 2 Duo) would not run 10.7. That would allow Apple to stop shipping 32-bit apps. The specific models that have 32-bit Intel processors are:
iMac: Rev A
Mac Mini: Rev A and B
MacBook: Rev A
MacBook Pro: Rev A and B
Except for the Rev B Mac Mini, these were all produced exclusively in 2006.
I expect 10.7 to continue to ship with both 32-bit and 64-bit kernels, but for the 64-bit kernel to boot by default on all Macs with 64-bit EFI. (By the way, I expect 10.8 to drop the 32-bit kernels and not run on Macs with 32-bit EFI.)
I expect 10.7 to drop all tools for Carbon development. (I expect run-time support for Carbon apps in 10.7.)
Long overdue and probably the most important new feature for 10.7.
HFS+ is really quite long in the tooth and Time Machine is screaming for a modern file system. If not ZFS, then another modern file system will be needed.