After a couple of months of testing it out here's what I'd like to see fixed (hopefully some of these will happen in 10.5.2):

Optimization/Performance:

Cover Flow can be slow sometimes and it happens to take up a lot of RAM if you look at a ton of files. It also makes Time Machine run very slow.

Ever since Safari 3 came out, it seems to take up a lot of RAM. A quick restart of Safari will fix this, but sometimes I leave windows open and I don't want to restart it.

Bugs:

Stacks can really be annoying at times (especially if you have dock magnification on). When you click on a fan Stack (say, Downloads) and you have magnification on, when you move your mouse up to click on one, the entire Stack shifts over.

RSS feeds sometimes just stop working in Mail. It won't show you any indication of this and it took me a while to figure it out. Most of my RSS feeds will have something new within an hour, so if I look at them and don't see anything, I manually update them, and it says that there's a problem with the site. You'd think that you could just restart Mail, but no, you have to restart the computer. This does get kinda annoying, as I hate restarting my computer, but it helps a little by giving me more free RAM.

I could just repair my permissions, through Disk Utility, to free up some RAM, but there's something wrong with DU since the update. I don't know what's wrong with it, but it used to take a couple of minutes, now it takes over 10!

Enhancements/Requests:

Right now the bottom half of Cover Flow is in List View. Ever since Columns View came out, I never used List View, as Columns View is a much easier was to navigate through files. Cover Flow plus Columns would be pure awesomeness.

I do like all the new features, and I don't regret upgrading, but I just can't wait for Leopard to be as stable as late-Tiger was.

Right now, I can only think of one app that is a bit buggy. That app is DragThing, which screws Spaces up.