In addition to the improved Stacks behavior (thank you, Apple!) already discussed here and there, there's a toggle for menu transparency, the optical disc sharing shown as a feature on the MacBook Air becomes a system default, and (ta da!) all hail the return of network drive Time Machine backup.
There's been some dark speculation that Apple deliberately disabled this feature from early Leopard builds and made it a Time Capsule only deal, so as to drive sales of same, but it looks like such speculation, she were unfounded.
Fixing Stacks and network backup are the biggies, in my book. Looks like 10.5.2 is shaping up to be quite the nice little update.
Getting the Stacks thing straightened out restores my faith in Apple's sanity.