Why is this great? For one thing, having your library synced and backed up to iCloud is a great way to keep your pictures secure and easily accessible; in addition, you should be able to save a good chunk of storage space by only downloading the photos that you need, rather than clogging up your device with every image you've ever taken.
Find my vacation
Having all those images at hand might make you nervous about ever finding anything again. Apple has anticipated your concern, though, by adding both a smart search feature and a Favorites section. The search field will initially prompt you with a collection of nearby photos, images taken at the same time last year (for nostalgia factor), and all-time favorites; but you can also search by date or time, location, or album name.
Favorites comes into play with your iCloud Photo Library: Under every picture is a small heart outline; tap the heart to fill it and add the image to your Favorites album. You'll be able to access that album on any iOS 8 device (or, in 2015, on your Mac), and make edits to images that automatically sync.
Apple's automatic smart albums continue to provide some use here, as well: The Camera Roll looks to have disappeared, replaced by the Recently Added smart album, which presumably collects images you've recently taken or added to your device. It joins app-specific and content-specific albums on the Albums page, along with the aforementioned Favorites album. Sadly, based on Apple's previews, there doesn't look to be any way to create smart albums yourself, and there's no indication that a "Screenshots" smart album is in the works.
An editing spree
In iOS 7, the Photos app offers a few basic editing features and filters, but for anything fancy, you need to download Apple's iPhoto app. In iOS 8, bits from iPhoto will make their way into the Photos app, offering you all sorts of advanced editing features.
Download an image to your device, and you'll be able to use a bevy of iPhoto-inspired features to crop, straighten, remove red eye, adjust lighting and contrast, and more. All of those edits sync across your devices, so that the fixed image appears in your library immediately. They'll also be non-destructive: if you decide you preferred your image unfiltered, you'll be able to revert it.
On top of that, iOS's new Actions options for developers means that your favorite third-party apps could provide filters and adjustments that you could use from within the Photos app just by tapping the Share button. Love filters from the Waterlogue or VSCO Cam apps? You may soon be able to use them without ever opening those programs on your device.
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