The biggest problem I have with my Galaxy Tab is sites that rely on mouse over messages and mouse over drop down menus. Since there is no mouse cursor, I can't activate the message or drop down. nfl.com is a good example of this. You can navigate to "scores" easily but getting to "standings" is problematic. All of the sites that rely on a mouse cursor or Flash can be rebuilt to support tablets but I'm not sure this is an improvement.
The lack of mouse over messages is a problem with icons as well. If I don't know what an ambiguous icon does, the only way to find out is to poke the icon or wade through documentation.