Here are my bugs/issues. My apologies if any of these are repeats.
Things that are annoying and should be fixed right away
-The non-standard open dialog to import non-steam games is a pain to use. Also, for some inexplicable reason, it shows invisible files. Please use the standard interface.
-Steam overlay for non-steam games is very much a hit-or-miss. In the non-steam tales from monkey island, the cursor doesn't show. In Halo, the game doesn't pause and the camera goes around and around. Doesn't appear at all in Starcraft II.
Bugs, but not annoying ones
--The text for achievements/friends who own your games when you mouse over them sometimes does not appear. When they do, it makes the drop shadow of the window do a funny dance. You can also mouse over them even when the window isn't active.
--The dock menu with all the added options sometimes appears, but usually it's just the basic one.
-Sometimes the steam window will flash when I dismiss a dialog
Cosmetic things that should be fixed because it just looks lazy/Windows behavior that needs to be removed
--Stoplight buttons do not obey system settings (if set to graphite, they remain colored)
--the cancel/okay buttons are in their windows positions. They should be reversed.
--Traffic light buttons appear everywhere, even places where they shouldn't. Following Apple interface guidelines, they should really only be present in the Friends list and main window. For the little dialog that appears when you get a message from a group, the buttons obscure the title
--When importing non-steam games, the icon is very very small.
--When changing an icon, in the file type it says "program files (.exe). Should be removed. Actually, this whole window should just be the standard OS X one.
--In the interface tab of the settings, "Select which Steam window appears when the program starts, and when you double-click the Notification Tray icon" should be "Select which Steam window appears when the application starts." Also, "Run steam when my computer starts" should be "Run Steam at login" This is more accurate and follows Apple's conventions.
--"shortcut" should be "alias"
--a weird grappling hand icon appears when you drag the lower-right corner. Not standard behavior.
--Steam overlay popup that appears when launching a game should appear in the top-right, to match the other notifications.
--The button to choose an avatar pic is truncated w/o any text. But hey, it uses the standard open dialog!
--The popup to choose your country is ugly and non-mac like with the scrollbar inside it.
--Content should be saved to Application Support, not documents.
-In portal, the windows-style open maps dialog has a button that says "Open in Explorer," which will open the Finder. Should be renamed.
Things that would be nice at some point in time
-That awesome UI overhaul that was posted earlier.
"Get Game Artwork" I have Tales from Monkey Island, but the non-steam version. It would be cool if I could select this to import the full Steam icon. Similar to Get Album Artwork in iTunes.
-Growl support to replace Steam's own notifications.
--dragging and dropping non-steam games to add them to the Steam interface would be terrific.
--Integration with crossover. So I could install a windows game, and upon launch it would automatically load in CX Games.