I guess the main concern is, the lack of apps, for example im an avid fan of tapatalk, but that particular app is not available for Windows. Its also quite a step to get a Windows phone, when Android and iOS has proven themselves as excellent operating systems.
If all you want your smartphone to do is make calls, mess around on Facebook, search the internet, check the weather, and get directions, a Windows phone is fine. But as you mentioned, the biggest disadvantage with WP (and Blackberry) is the lack of apps, and to another extent, the delay of the apps that are available. You're used to an Android. If you expect to play the newest, hottest game, you'll be regretting buying a Windows phone. Before getting a Windows Phone, you should see if the apps you use on your Android are available on WP, or at least have an acceptable alternative. If you must get a Windows phone, go with the 920 over the 820 (I'm assuming you have AT&T). Yeah, it'll cost $100 more, and it's slightly bigger, but the screen is far superior and the camera is better.
Frankly though, I really don't see any advantage of getting a Windows phone over an Android or an iPhone. Android vs iPhone arguments aside, there's so much more available on those 2 platforms that WP can't compete with (at least, yet). iPhones are easy to use and have the most apps available, and usually sooner than Android. Android has greater customizablilty and some functionality that iPhone doesn't (especially if you root it). Plus the Google "experience" (gmail, Maps, Street View, etc) is the best around. Depends on what you want. But I honestly don't know if there's anything a Windows phone does that an Android or iPhone can't do, or anything that it does better than what those other two already do. Office, maybe, but of course there's apps for iPhone and Android that can do the same well enough. Maybe it's the simplicity and functionality of the WP8 interface you like. Well, you can download a launcher interface for your Android that looks and works exactly like it. Nokia makes a tank of a phone though, so outside of the OS, that might be a consideration.
My advice, stick with Android. If you don't want to spend the $200 (or more, for more storage) on a Galaxy S4 or HTC One, the S3 can be had for $100 and is still a fantastic phone, as is the LG Optimus G (which I'd personally choose over the G3). Or for $.99 you can get the Galaxy Express or HTC First (just disable the Facebook crap). The First is probably the better phone (better construction, much better screen), but doesn't have a micro SD slot, so you're stuck with the 16gb (only 12 available) it offers. The Galaxy Express has less built-in storage but can take a micro SD card. Depends on what you want to load it up with.