For many millions of people, Android "just works". It also has more apps than iOS and WP.
Smooth? Tell that to people with iPhone 4 on iOS7. Even my iPad 4 stutters on iOS7 from time to time.
We've been hearing that many new/critical features will be added imminently since WP7 in 2010.
I highlighted the biggest plus points of each OS, having decent experience with each OS.
You can't say that iOS isn't smooth if to use a 2013 version of the OS on a device released in 2010 (i.e., the iPhone 4). I'm sure that would be the case on most if not all combinations of mobile operating devices and mobile devices.
Yes by numbers, Android has the most apps by number. But how many of those "apps" are themes, extensions, add-ons, etc.? And when deciding on an OS to use, I don't understand why people concern themselves by the number of apps available to use on the OS. Ask yourself how many applications do you use on a daily basis, or even a monthly basis. Thinking about that will quickly diminish the significance of the number of apps an OS has to offer. Other than games and OS specific utility apps, IMO each one of the three OSs has a pretty much equal selection of apps that would be needed by users on a day-to-day basis.
Like you said: "We've been
hearing that many new/critical features
will be added imminently since WP7 in
2010". See why did I italicize those words??? WP is still an evolving OS. It still lacks some basic features that iOS 7 and Android have. For instance, you can't toggle among "Ring", "Silent", "Ring Vibrate", or "Vibrate" without going to the settings -> ringtones -> then toggling vibrate on/off. There is no quick toggle to turn wifi on/off as there is on iOS and Android. To change the brightness I have to go to settings-> brightness then choose between low, medium ,and high on my Lumia 920. Those are the only three brightness options.
These are minor features that WP is lacking. But then again, do they make the OS unbearable? Absolutely no! It's a wonderful OS to use that is still rapidly evolving compared to Android and iOS, both of which have matured for the most part. Need I remind you that you couldn't change the homescreen background on iOS until iOS 3 (or was it iOS4).
As for what original question by the OP or any other person deciding on which device to purchase. I would look at which OS fits the best for what YOU are looking for.
Personally, I would go with the Moto G simply because it is the newest device. Second choice would be the Lumia 920. But if deciding to go for a WP, I would probably go for a new Lumia 525 or 521 as opposed to a used Lumia 920. The iPhone 4 is ~3.5 years old. IMO in terms of having a relatively future proof device, I would not go with the iPhone 4. Same goes for the Lumia 920. Although it is not as old as the iPhone 4, it's been over a year since it was released. I'd imagine it will stop being supported by MS/Nokia (in terms of OS updates) after the next major update. With the Moto G, you will have the latest version of Android available with a strong developer community behind the device (to support custom ROMs for the next few years).
I have experience with each of the three operating systems. I currently own a Lumia 920 and a 2013 Nexus 7. I have no regrets with my decisions to purchase those devices as each serves a different purpose for
me. Find something that works for
you.
Op, if you would like to know more about my experiences about the operating systems, I can provide an unbiased perspective. Let me know.