(Mostly negative, good things are in bold.)
Number one: tabs across the top? Don't like'em. The rationale you hear is "but that makes the URL box part of the page, which makes sense." But then it also makes the bookmarks bar a part of the page, which it isn't... so it's B.S. Why don't I like it? 1) I have no problem with the concept of the URL box. I've been on the WWW since 1995. I don't need any hints about what this box does. And for me, it makes perfect sense to see the tab directly connected to the page's content. 2) It looks cluttered. And when I want to grab the title bar to move a window, I now have to be careful where I grab it lest something undesirable happen. [Later] The more I use it, the more I dislike it. With a few cascaded windows open, and multiple tabs in each, it really looks messy.
Speaking of the URL bar, I like a standalone "reload" button, not a shrunken one tucked into the corner. Being a web developer I use the reload button a lot. And the "New bookmark" button being tacked onto the URL bar looks kind of ugly.
Other gripes about the new tab system: I prefer each tab to have a default width, getting skinnier as needed, rather than the new full-width/50%/33%/25%/20%/etc scheme it uses. Camino used to do that. (Maybe it still does, I don't know.) I didn't like it 5 years ago and I don't like it now. Left-aligned + standard widths = consistent placement == a good thing. The "lock" icon in https tabs also adds to the clutter, and I don't like the "close" button being hidden except for a) active tabs (which, strangely, includes the active tab in an inactive window) or when you hover. Hiding UI elements is almost never a good idea. Also: diagonal lines mean "resize," not "move."
Waah! I thought there was some kind of "tabposé" or CoverFlow feature for open windows/tabs but the only time you get the cool grid is for the main view. Which I hate. It's a waste waiting for it to load, and they move around based on usage, so what good is that? (Update: you can click 'edit' and then 'pin' certain sites so they don't move around.) Also: 12 sites, no matter how big or small your display is? Dumb. (Slightly less dumb: click 'edit' and you can change this to 6, 12, or 24.) In any case, I don't like this feature in Chrome and I don't like it here. As for CoverFlow in bookmarks and history... eh. I never use bookmarks, and when I'm looking at history, good text searching would probably be more useful. (Which it apparently has. I'll have to check that out.) And I like that you can set your history to be longer... mainly because (as far as I know) the "Restore All Windows From Last Session" option works on history. I've used it several times when a weeks-long browsing session ends and I've lost plenty of windows. (Unlike when I was using Saft, which always worked.)
Safari 4 is fast. But I still prefer an actual status bar to just a "something is happening" spinner. And, actually, speed has its price. I'd rather have a short delay and a nice page than the "jittery" effect you get with Safari's new way of drawing each DIV or TD as it gets it and then moving it a bit (sometimes repeatedly) as new content comes in. Opera works (or worked) this way and I never like it there either. Draw content in chunks and shift it as you need it... but not multiple times in less than one second.
I don't much care for the Awesome Bar in Firefox and I don't care for Safari aping it here. Their implementation of it is cleaner than FF or Chrome but I still prefer the old system better... especially since their algorithm for deciding how to order the sites (weighted based on frequency of visits and time since last visit, and the include bookmarks too) is great.
Happily most of what I don't like can be turned off in preferences or with the commands shown here.