Windows is moving away from the OS so many people are familar with. For Apple users, it will be like the move from OS 9 to OS X, a dramatic change. If what I write seems a little awkward, my apologies, but there is a lot of information out there.
Apple: When Apple developed the iPhone, they created iOS to work with it. They also had been working on the iPad for almost a decade, so they knew it was in the pipeline. Therefore, they designed iOS with the ability to move to a larger format. Decades of experience with minimalist screen design worked for them in developing iOS. With iTunes already a big thing with the iPods, it was easy to decide to go with the app approach rather than widgets. Apple also recognized that one of the headaches of computer systems was the risk of downloading software from the internet. That was a main reason they went with a closed system. Apple is a control freak, but in some ways that works to our advantage. You do not need a lot of visible security at a beach party, but you want it at a nuclear power plant. Where your mobile device fits between those two extremes, can vary very much, depending on personal opinion.
Microsoft: I do not know a lot about Win 8 yet, but it will be designed from the beginning for those who use Windows and are familar with it. Microsoft's approach is different. They will have a mobile OS and a desktop OS. Where they differ from Apple and Google, is they are going to make the tablet version a derivative of the desktop OS. The idea is that people are fustrated with the lack of being able to use MS Office with their tablets and want a more full featured experience. In theory, it sounds good. The problem is that the system will require a much more powerful processor, lots more storage, and other amenities. My personal thought is that tablets are not ready for that and battery life will suffer. It will be interesting to see how it plays out. I doubt that it will be a neutral subject. I forsee some of people loving it, while others hate it. It will probably be geared to commercial users, and be too expensive for the average person. My big concern is that MS will leverage their lock on business and government users with MS Office. If they make Win 8 the only way to get Office on a tablet, they could force a lot of companies to forgo other devices. However, MS has lots of experience with skirting anti-monopoly laws and could bring out a iOS version that is just enough to keep the regulators at bay. I would also expect that MS will maintain a locked down OS for security reasons. At the very least, expect apps to require a security certificate before they can be downloaded or used.
Google: When the smartphones began to hit the market, Google wanted to become a big player. Without any real experience in designing operating systems, they decided to take an old free universal OS base (Java) and make it into a mobile system. Java is a very old format designed for web use, and not the easiest program to use. And not every needed component was included under the open license provisions. While Sun Microsystems was not interested in patent protection, Oracle has bought them out, and they are interested in receiving patent royalties from Google. Google is gaining on OS development, but it is like updating a mobile home. You can do a lot, but you still are going to have a poor foundation that is not going to hold up in a crises. One thing that I really dislike about Android, is that in many cases, if you buy a device, you will not be able to use another version of Android. Plus the Android experience is different on each device, because the OS has to be modified for each device manufacturer. A few other items: Android devices usually use 16:9 screens as opposed to the 4:3 iPads. Great for watching videos, but poor for most everything else. Plus, you can have a larger rated screen that has less area. A 10.1" screen at 16:9 is almost 10% smaller than the iPad 9.7". My biggest gripe is that you do not have a central ecosystem to deal with. With iPads, Apple takes care of software and hardware issues. If you have a problem, it is easier to find a dedicated repair person for the iPad than for any other tablet. With the Android, or other similar devices, if you have a problem, you can get into a tangle between software and hardware. What do you do with an open device that crashes due to an app you downloaded? It is not as bad as it used to be, but I remember the early days when you might have several vendors to deal with and no one would take responsibility for failures. It was like buying a car and the engine, transmission, radio, tires, and bumpers all had to be handled by their own vendors. It was those types of problems that lured me to Apple twenty years ago. It is much better now, but there are still issues with a lack continuity between software and hardware.
This is just my way of seeing the issues between the three main competitors in the tablet market. If I was Google, I would be working on an OS that does not use Java as a base. However, I do not think that Google has the ability to make the long term commitment needed to make it happen.