Insignifant annoyances here. Spaces, meh, I don't see the need. Stacks, ANNOYING as hell, until I got a way to make the applications stack look like my old Apps alias on the dock. It still needs one more click to get to the app you're wanting if you use the stack on the dock. I miss the right-click option on Stacks. Spotlight would work well as a workaround. I can see me using spotlight more now, for launching apps.
I miss the animated horizontal candycane when booting up. Seeing nothing but the blue screen makes me feel like something's wrong, even though it's not. I'm a visual person. I need to "see" something happening, even if it's just eye candy.
Time Machine alone is worth the upgrade. The other extras are just bonuses, IMO.
Sure some programs don't work, but that's what the desktop running Tiger 10.4.10 is for. Networking has a nice new GUI, that works fine. Flash works fine in both Firefox and Safari.
Safari plugins don't work yet, but will soon.
Again, I'm lucky I have another machine to use for the "must have" apps that aren't working with Leo yet. So far so good, despite the glitches.