So I've listed my favorable first impressions about Leopard. Unfortunately Leopard isn't perfect, and there are a few things that really bug me.
.Mac seems to have taken a stronger role. By default the .Mac sync icon was stuck in my menu bar, even though .Mac was turned off. cmd-drag got rid of it. The problem? I still get .Mac errors about syncing my address book and things. I really could do without the random dialog.
Firefox menus. The bookmarks sub folders get more out of line as you go down. As in the little arrow points to empty space, and the box is below the line you've highlighted. This is likely more on Firefox's side, but it is kind of a strange thing to break. While I'm at it, Firefox sure didn't handle the upgrade well. Quicktime videos don't play with audio, Flash is acting "funky", and it crashes as frequently as even. If only I wasn't addicted to Quick Searches.
Terminal tabs are moved between with cmd-shift-] (ie cmd-}). Awkward.
Cover flow in finder gets a big "meh." The load time on my mini for a folder of pictures makes it less than stellar. Much like when I cover flow all of my albums in iTunes.
The way stacks show what is in them can be a little weird. To someone who isn't me (like the Ars Technica guy) this would be frustrating.
On the note the Ars Technica Review, I have to say I'm indifferent about the dock. I might change it, but I don't really care. The new folder icons I think are quite clear even in column view in Finder in contrast to his opinion.
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Re .mac, I don't see what you're seeing. Yes, it was in your face during the install but afterwards there's no trace (except the icon in system preferences and probably annoying menu entries in programs like iTunes and iPhoto)
I solved the terminal tab keybinding problem, see my blog.