Here are some OS X -related bugs I've found and which I think don't apply on Win/Linux.
Copying text from windows doesn't work as shortcut - The normal shortcut in OS X is apple+c, not ctrl-c. Copying from input-field works so I'm guessing you have some hack for the windows' panels. Copying works with RMB and selecting copy from the popup. (ver 1.61)
On a related note selection disappears when new text comes. This might not be OS-dependent, but it's still really annoying when you want to copy something from the main window. Especially when mouse wheel doesn't turn scroll lock on. (ver 1.61)
Opening URLs doesn't work - Clicking on URLs doesn't do anything. Tested with Firefox and Safari as default browser. (ver 1.61)
Text fields in channel popup-menus don't work - Such as "rename tab" and "add channel". You just can't write anything to the text fields. So configuring the windows has to be done by hand to the config-files. (ver 1.61)
Alpha-setting for news-window doesn't save - This might not also be OS-dependent but still a bug. I'd like the window not to be transparent and it always is when it opens. (ver 1.61)
Turning alpha off from keybindings does something very wrong. This makes the window all white-bright grey and totally unreadable. Definitely different than alpha 255. Same with script editor. On help-windows the alpha setting doesn't seem to affect anything: you can't turn transparency off. (ver 1.61)
Starting the client asks always about some certificate - "This application was signed by Mythiscape, but Java cannot verify the authenticity of the signature's certificate. Do you trust this certificate?" Might not also be OS-dependent, yet annoying. (ver 1.61)
Seems like I have also these non-OSX problems here quite much, but don't feel like starting a new thread about it. So, scrolling command-backlog doesn't work logically. If I enter some command and then press up, that command comes to the input line. Ok, perfect. But if I press enter and then up, the selected command is second to last, not last as it should be. (ver 1.61)
Resizing realm map -window doesn't work, and I can't get the sidebar with area names go away. Also as improvement it could have a "center on me" -button, because usually it would be faster to zoom to a good level and then move the window to the correct location. With center on movement turned on this is of course irrelevant (that's a great addition btw). (ver 1.61)
And as a cosmetic note, client's icon in dock is a generic java-application icon and not the cool batclient-icon, which is on the .app-package. Also if you have a shortcut for the app in dock, starting the client wouldn't show as an arrow in this icon, but as a new generic java-icon. (ver 1.61)
I'll add these here I remember/find out others. I have to say that all in all the client works very well and these affect actual gameplay very little if at all. Good work team!