I've heard Apple changed the 'stoplight' buttons at the upper left of windows. Are they vertical again?
No, still horizontal, but in many windows they're now integrated into the toolbar, rather than above it, thus saving some screen real estate. They also have the same flatter appearance as the rest of the operating system. If you're a long-time Mac user, the experience will be a bit odd at first, but it should be a nice change — especially given the widescreen aspect ratio of all of Apple's current displays.
In addition, Apple has tweaked the behavior of the green button. Previously, it's always been a zoom toggle — though exactly how it zoomed varied widely. In Yosemite, the green button is now a full-screen toggle: individual windows zoom to the width of the screen when pressed, while the main window of an app enters full-screen mode. Many of us will miss the green button's zoom control, but given Apple's current emphasis on full-screen mode, this change makes a lot of sense and reduces clutter in the toolbar.
Are full-screen apps dead? I don't see the little double-arrow icon in the top right of windows.
Full-screen mode is alive and well. But as we just mentioned, Apple has done away with that specialized double-arrow button and instead changed the functionality of the green 'stoplight' button. The Full Screen keyboard shortcut (Command-Control-F) still exists and works within supported apps (Safari, Mail, and Calendar, for example). When you hold down the option key, the green button will turn back into its more familiar function of expanding the dimensions of your window.
What's this new Dark mode they mentioned in the keynote? How does it look?
The preview version of Yosemite we've seen doesn't currently offer Dark mode; chances are it'll appear in future betas. As you say, Apple did preview it on stage at the keynote: it appears to darken the menu bar and associated menus, with light text on a dark background.
How much does Yosemite resemble iOS? Does it still feel like prior versions of OS X?
While Yosemite does, as we mentioned above, pick up some visual effects — the translucency, brighter colors, flatter icons, and typography — from iOS 7, it does so in a way that feels very much like an evolution of OS X, rather than simply aping iOS. There are some options in programs such as Safari (the Favorites bar, for instance) that are hidden by default, but they can be easily accessed by power users. And as we noted earlier, if you really dislike translucency, you will be able to turn it off.
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