iCloud Drive makes all of the files that you have stored on Apple's cloud servers searchable and taggable from the OS X Finder. Auto-syncing across devices, your web connection will be vital as your offline and online files get jiggy together.
It all sounds sneakily similar to Dropbox. iCloud Drive syncs your stuff across all your devices, and across all platforms—even Windows. Which means, with Finder integration, iCloud is better. And that's a good thing. Although, the main takeaway here is that iCloud is sort of mimicking what Dropbox and Google Drive and whatnot already do automatically.
Also worth mentioning is Mail Drop, which sends large files through iCloud instead of getting bounced back, up to 5GB. Pretty useful stuff.
Photos are also benefitting from this iCloud refresh. Now all of your photos will be available on all of your devices. Search is improved too, based on location, time, albums, and so forth. iCloud Drive also syncs the edits on you photos. Crop a photo on your iPad and hit save, and the edited pic will show up on your iPhone. Synergy!
From OS X Yosemite to iOS 8, here's everything Apple announced at WWDC