1) Best 'experience' from a smartphone. The Live tiles make your home screen more interactive and provide more feedback/information than any other smartphone. The phone is 'alive' where other smartphones are dead.
2) Super quick OS. Even running on inferior hardware the Windows Phone OS runs smoother than the competition. If you think of the OS as the foundation, it's the strongest foundation of any OS.
3) Microsoft ecosystem. My wife and I both use windows phone 7.5 devices now (and getting 2 new lumia 920's on friday). We have all of our contacts, emails, calendar, social life sync'd up online. The shared calendar is 100% seamless making it extremely easy to coordinate events/etc with each other (or even leave notes for each other when we are apart! - Put something on my calendar either via my phone or email and it shows up on her phone immediately!). With the introduction of "ROOMS" on the WP8 it even improves on this experience!
4) The Apps! Tons of apps, we keep finding new apps that we didn't know we needed.
Also, the phone integrates seamlessly with the Xbox 360, which is the only media device connected to our TV. We don't have cable tv, but thanks to the Xbox we can still watch netflix, youtube, HULU, Amazone Prime (instant movies), iheartradio, last.fm, etc, etc, etc. And all of that can be controlled via our Windows Phone(s).
5) NFC and the Wallet app. Now Windows Phone 8 almost completely replaces my physical wallet. I really hope the NFC technology grabs hold of the retail industry. Imagine not ever needing to carry a physical wallet anymore (other than for your ID) and being able to make all your purchases using your smartphone.
6) Kids Corner! Yes, seems a little gimmicky maybe, but seeing as how every single one of my friends is married with young kids this is a huge plus. Anytime I'm hanging out with my friends and their kids, the kids want to play with their parent's smartphone. I recently talked about the Kids Corner of Windows Phone 8 with some friends and they thought it was an excellent idea (and wished their phones had this!). A lot of complaints come from their kids messing up their phones when playing with them(which is often), but this wouldn't happen if they had kids corner.
I could go on and on.
EDIT: I forgot about the Skype integration. Being able to receive Skype calls even with the app turned off, and answer these calls just like you would a normal phone call is an awesome feature. I believe this means you can now makes calls via wi-fi and not be forced to use your phone data plan or minutes for skype calls (which also means you could use your phone out of country and still make calls for free!). Combine all of that with a forward facing camera and you are left with the ability to skype calls to any skype enabled device anywhere in the world.