Collaboration and iCloud
Collaboration is a new feature with the latest iWork apps, though it's something that's been around a long time in other office suites like Microsoft Office and Google Docs. With all three iWork apps, you can invite up to 20 people to work on the same document simultaneously and you'll see other users interacting with the document in real time. The app color codes each user so you know who is working on a particular section. People also can make comments on sections that show up for all people sharing the document. These are things that Google Docs already does, but it's nice to see it added for Apple devices.
If you just want to share your presentation with others, you also have the option to touch the share button on the main presentation index screen to share a link via iCloud or you can send a copy of the whole file via iMessage or e-mail. There are also options here for syncing your presentation to iTunes or to a remote server via WebDAV.
Another useful feature that comes with making the iOS and Mac versions (mostly) the same is that your work is synced across all devices through iCloud and you don't lose your formatting or layout when switching. This means that changes you make to a presentation on your iPad will be reflected in the same document on your Mac, iPhone, or even the beta Web version of Keynote you can access on any platform with a Web browser.
Some notes about the Mac version
All of the iWork apps including Pages, Numbers, and Keynote, were rebuilt from the ground up to share the same features across Mac and mobile so you could do the same things on any device, but in that process, the Mac version ended up with fewer features. There are already a ton of complaints on the Web specifically from Mac users, and they have a point: The new version of Pages for Mac doesn't have as many features as it did in iWork '09.
Apple has recently addressed those concerns in its support pages, saying that many features -- new and old -- will be coming to the Mac version over the next six months. As I mentioned in the Pages review, Apple says this release is a starting point, but there are more improvements coming including a customizable toolbar (like you had in iWork '09), the company is bringing back transitions and builds from the old version, improvements to the presenter display and AppleScript support, and more. You can read a full list of changes across all the iWork apps on Apple's support page.
So, the bottom line is, if you use Keynote on your Mac, I do not recommend you pay for the app unless you can download it for free on a new Mac. Otherwise, stick with iWork '09 until Apple makes the promised changes to all versions.
How does it compare with Google Docs?
Now that the iWork apps are free, there's going to be comparisons with the Google Docs set of productivity tools. They both have spreadsheet, presentation, and word processing tools, and sync across devices using their own cloud services (iCloud and Google Drive). But it's important to note that the new iWork apps are only free for people who buy new Apple devices, or those who have already bought the earlier versions of the apps. In other words, people who don't have a new device or the previous versions of the iWork apps will still have to pay. The new versions are useful on iOS and I still recommend you get them, even with the steep $9.99 price tag. It's worth it for the templates, the touch-friendly interface, and the Apple ecosystem that works seamlessly across apps and devices.
With regard to Keynote specifically, there is no replacement from Google Docs on iOS devices to make presentations. In fact, the new iOS version offers more themes, more and smarter transitions, and a more elegant interface design than what you see on the Google Docs presentation tools even on the desktop. In other words, for building presentations on either desktop or mobile, Keynote beats Google Docs pretty handily.
Keynote for iOS is a step up from the old version, with new themes, animations, and transitions, not to mention a redesigned interface that offers a better layout than what we had before. Putting the iOS and Mac versions on a level playing field means that your presentations are always synced to every device, and your presentations will work well regardless of which platform you're viewing them on.
Unfortunately, Mac users will not be as pleased with the new version because much of the functionality found in the old desktop version is no longer there. Like I mentioned earlier, Apple is bringing many of the most popular features back over the next six months, but also let people keep iWork '09 for the time being to keep that functionality (in other words, this update doesn't replace your old installation). It's disappointing that the Mac version was effectively downgraded with the new version, but it's good to see that Apple is paying attention to user complaints and working to make the apps better. In other words, for now, only get Keynote for Mac if you can get it for free with a new desktop or laptop and wait for Apple to add to the current version before switching over.
Still, if you primarily use Keynote on your iOS device, the new version is a significant improvement over the last. Though it's not free for all like Google Docs, it offers more tools for presentations across all your devices. Eventually, when we all move on to new iOS devices, Keynote will be free for everyone, but in the meantime, it's worth it to pay for this update on iOS for a marked improvement over the last version.