Here's my list...
1- I know Apple sanctioned themes will never happen, but since the Metal GUI is so prevelant now, why not give us the option to use it system wide? Let us choose between Aqua or Metal for the entire UI and apps.
2-Open/Save dialogs need to be beefed up. Borrowing some ideas from Default Folder would be nice (ability to rename, move, copy, etc), as would having addt'l options of arranging files in open/save boxes, like you were able to do in OS 9.
3-Consolidated appearances. It makes no sense to have your appearance settings divided between three system prefs - Desktop, General and Dock. They should all reside in one "Appearance" pref (along with the ability to change between Metal & Aqua).
4-Give us the ability to turn off "tool tips" for the dock. I'd also like the option to give them a background color, so they stand out better against applications it obscures. The dock works fine for beginners, but it could use some advanced features (that would be turned off by default) that would make it more configurable.
5-Jaguar went a long way to reducing the spinning beach ball problem. Let's kill this phenomenen once and for all in Panther.
6-Borrow from WindowsXP and allow for a "quick logout", which leaves a users programs running while someone else logs in. It's a cool feature, and if WinXP has it, so should OS X.
7-Better Finder customization. Take a cue from Path Finder. Give us the ability to change typeface in all views, background colors in all views. Bring back row seperators, etc. Better yet - buy Path Finder from Cocoatech and use that as the basis for Panthers Finder. This would also return Finder Labels to the Finder, which is still sorely missed.
8-Fix the Carbon toolbox so all apps (carbon & cocoa) can read/write 256 character file names.
9-Fix FTPing from the Finder. I've yet to get it to work.
10-Give us more options when creating PDFs from the "Print" dialog box. We should be able to control the level of compression. Otherwise, it's an almost useless feature.
11-Give us a real means to lock down our computer - not this half baked solution of enabling a screensaver with password protection. That eats up CPU cycles. I want a basic dialog box that says "this machine is locked" with the desktop showing from behind. This will allow me to start a render, lock my machine and go to lunch, and not lose CPU cylcles to the screensaver.
12-Give us back the ability to print finder window contents easily. This shouldn't require a 3rd party utility or having to produce a screen capture to accomplish.
13-Bring back "Single Application Mode", a feature that was touted in the Public Beta. Allow it to be turned on/off from the Apple Menu (this in turn could be enabled from the "Appearance" system pref.)
14-iChat - give us the ability to video conference with it.
15-Screen Effects. It would be nice to have a screensaver that allows you to choose a graphic (PDF,TIF,JPG), select some text, and let the items float around the screen. Much like the Apple screensaverr, but with customized text & graphics.
16-Enhance TextEdit. It will already give you a WYSIWYG preview of HTML text. Go one step further and make TexEdit a HTML WYSIWYG editor. Nothing complex like Dreamweaver, but some basics WYSIWYG tools (type styles, img, tables, etc). The Public Beta of OS X came with a alpha quality HTML tool that could serve as the starting point.
17-Simplified Font Management. Auto activiation/deactiviation. 'Nuf said.
18-Replace the built in Help systems HTML viewer with WebCore. This should improve the Help viewer and speed it up ten fold.
19-Give us a basic Backup tool. Something that doesn't require a user to be a .Mac subscriber, and doesn't require 'net access to check if you are or not.
20-Basic HD defrag utility. Again, Windows has had it since Win98, why doesn't OS X?