Quick Look is the Best Computing Innovation. Ever.
I finished a clean (erase) install a couple hours ago and have been using Leopard (with nary a problem) since then.
I was playing with Time Machine, spaces (having four screens on two monitors is greatUbuntu linux on one, Windows XP on another, Web dev apps on another, Debian server on another, and more screens to spare is GREAT!), and all the other headline features, but one feature is completely amazing in its utility and execution: Quick Look!
Quick Look is always instantaneous (at least, on all the Macs I've tried it on so far), and it recognizes most any file. And being able to instantly go through a ton of files, previewing each one how you like (small or full screen) is genius! What a great feature.
I was worried by some other posts that the transparent menu bar would be annoying, but it really isn't at all.
Also, the new iChat is EXTRAORDINARY. Screen Sharing is worth upgrading for me, because I already shared with two guys today and showed them how to set up their Leopard Mail installation on our network!
And the dock's reflectivity is a nice touch. Everything is so polished!
Finally, everything (and yes, I mean EVERYTHING) is faster. Photoshop, Word, Safari, Mail, iCal, Dreamweaver, Parallels, iPhoto, iChat, and Address Book, to name a few apps, are all noticably faster. Do I regret the upgrade? Not one bit. I'm loving it so far! The intro video was icing on the cake; does anyone know where that actual video file is? I'd like to see it again!