Here are some of the key differences between Android and WP7:
1. Notifications
In Android, your toast notifications collect in a centralized location so you can view them whenever you want. In WP7, if you miss a toast (pop up) notification you can't view it anywhere. Instead, the live tiles on the home screen display this information. (ie messaging tiles displays number of new texts, etc) Some tiles will flip to give you updated information. You can pin any app you want to your home screen to make a live tile for it, and swipe to the right from your home screen for an alphabetical, searchable list of all your apps.
2. Google Services
Microsoft's built in search engine is Bing. If you want to google search, you will need to use the web browser, which is IE9, or a third party app. Apps exist for some google services, like Gtalk and Google Voice, but you may find their functionality somewhat limited. Browser-wise, Chrome and Firefox aren't currently available. Google Calendars and Gmail contacts automatically sync with your Gmail account from the moment you add it to your phone, and Gmail is seamlessly integrated, and live-tile and push-supported. (MS Exchange support is also seamless)
3. App Selection/Functionality
As you are aware, WP7 has fewer available apps than Android. Notable apps currently missing include Pandora and Skype, among others. Spotify is present, however. MS is committed to bringing the top 25 apps to WP7, and is going to great lengths to assist developers, including free devices and development software, as well as assistance learning to use dev tools. A Skype app is coming within a couple months, and full integration with the OS is coming with the Apollo OS update in Q4 2012, just in time for the end of the world.
In terms of functionality, the apps you're used to on other platforms look different: in order to promote a consistent user-experience, WP7 apps utilize WP7 "Metro" interface and gestures, and the options and other feature will be in the same place as on other apps and the native programs. (ie: in-app search button will almost always be in the middle of the bottom of the screen) Also a couple apps are missing functions present in other versions.
4. 3rd Party Keyboards
If you like Swype, you're out of luck. Good news is, WP7s native keyboard is fast, accurate, and has top-of-the-food-chain prediction/correction. Even the most committed Apple and Android fanboys admit we have them beat here!
5. Multitasking
WP7 supports a "quick resume" model, similar to iPhone, rather than true multitasking. A few apps, like music players, will run in the background, but most will "pause" when you leave them. Holding the hardware back button will bring up card views of your recently paused apps, and you can pick up where you left off. The plus side to this is conservation of resources. This means your phone won't suddenly lag, and you won't need to use an "app-killer" to save your battery. WP7 will automatically kill background tasks as needed if you're running low.
(note: I copy/pasted this from an email I sent to a friend of mine thinking of making the jump to WP7 so some of the info here might not concern you.)