I do not believe that the new features will "POOF" make the bugs disappear, but it is theoretically possible. As I do not actually have a Leopard preview (read: I don't have $500 to spend for it when primarily I develop web applications and command line applications), I cannot really tell.
I just think that it is likely that the not actually very extensive list of bugs has either been (mostly) fixed in one of Apple's own forks, and just not merged (perhaps because it would cause danger of releasing secrets), or else are easy to fix, and Apple just hasn't done it yet, prefering to focus on other things.
However, it seems (from what I have been reading) that Apple has not really been working all that hard on the seeds which have been shown to developers. They have probably only been giving developers what they have needed, for example:
Time Machine: Applications should integrate well with this. See the preview on Apple's website - doing integration with iPhoto like that takes coding not just in Time Machine but in iPhoto as well.
iChat: There is some integration with that iChat theater feature that developers can work on
iCal: There are apparently some new features here for querying, etc.
Dashboard: One word: Dashcode
Spotlight: No new features except some new integration things, I believe, and more search capabilities. Nothing exciting, so why hide it?
Accessibility: Programs need to use accessibility features to be accessible.
64-bit: Self explanitory.
Core Animation: This is pretty much Only a developer's feature. The developers use it to create great things for us (read: me!)
Mail: This one I'm not sure what the improvements are needed for for the developers
Safari: The tab moving-around stuff and help and search stuff are probably meant more to give developers ideas (and why hide it - it isn't that much). After all, what good is Core Animation if you have no idea what to do with it?
ZFS File System: There are probably integration issues for developers.
In fact, I think Apple may be not fixing bugs in the developer previews because they have been burnt by over-enthusiastic merges of code before. I believe it is likely that the iPhone "hints" we got through string resources in iTunes came from such merges, when a developer accidentaly merged information pertaining to the iPhone.
I bet Apple has fixed this mistake by keeping their forks for Leopard almost completely separate, only merging when they absolutely have to.
Now, since I have given my thoughts on what the practices of the Leopard developers might be, I can now offer some speculation on things which might be done:
Zoom: Based on some of what I saw on the ADC website, it looks like only a few lines of code would be needed for Apple to modify that little green zoom button on every window to not just suddenly change the size of a window, but instead do it gradually, using (you guessed it) Core Animation.
Close: Could we see some animations on closing windows? A fading-out? A whirlpool/wind? Something like when you press "X" for a widget on Dashboard? Again, would probably take very minimal coding on Apple's part. Would be almost tragic if they didn't do it! Though, they may not have thought of it...)
New User Interface: I think we might see a movement towards a UI which looks like iTunes does. iTunes is installed on more computers than any other piece of Apple software, so it is likely enough that we can look to that to demonstrate how future UIs from Apple will look. iTunes 6 was an accurate precurser to iLife 6, after all.
The first two I wouldn't count on, even though they would probably be exceedingly simple to implement. (Though I am not sure about that zoom button - that might actually be implemented through software, which would make it the developer's choice on whether or not to add any neat effects). The third, I give it a 70-30 or 80-20 chance that it will happen.
TV is "due" March 20th'ish ( said with humor ).This also gives devs like say for example Elgato,inc. the chance to "intergrate" their app with the new Front Row.This is why there is a new spotlight.