I mean that things just don't look or act right. Try firing up Quartz Debug (if you have a Mac), setting the UI Resolution scale factor to 2, and restarting some apps. Every app I checked has problems.

For example, every app that used a standard Mac toolbar draws it incorrectly. Address Book's split views do not resize correctly. iCal's colored checkboxes are incorrect, and the popout from double-clicking a calendar item is very broken. In Safari, the busy icon on tabs isn't drawn correctly, the Top Sites search box is clipped, and the Bookmarks bottom toolbar is drawn incorrectly. The Dock puts right-click menus in the wrong place and draws them wrong, and handles drag and drop incorrectly. Finder doesn't scale the desktop icons. Preview's notion of "Actual Size" doesn't honor the UI Resolution scale factor.

Outside of Apple apps, I find that Photoshop ignores clicks on the menu bar entirely. OmniGraffle's toolbar is extra-broken, the disclosure triangles in its palettes don't work, and it draws the contents of documents incorrectly.

World of Warcraft scales up more than 2x. I can't even get to the username/password boxes. (This could be a feature.) Sketchup draws my document in the lower-left quarter of the window and random junk from the WoW login screen on the other three quarters.

Contrast this to the iPhone 4 experience. Every app I've tried looks at least as good on iPhone 4 as on iPhone 3GS, and none has had any drawing or behavior defects.