I have been using Windows Phone 7.5 - Mango(HTC Mozart) for past few months and I just love it:). Here are some of its features:
Live tiles - Single location to find all updates/news from office/family and friends etc. Customize it as per your needs. Set it one time and it keeps you updated at all time.
Groups – Makes it easy for you to mail/sms/chat/message all your related contacts in one shot. You can even switch between IM/sms and Facebook chat simultaneously.
One touch 8MP camera – Autofocus 8MP camera. I feel it is better than my Canon;)
Voice activated search(Bing) – Search is now available for anything and everything on your phone(contacts, mails, location, shops, maps/directions, music, web., etc etc etc)
X-Box Live – This is one of the coolest features that I love. You can now link-up to X-box live, create your own Avatar and play games online with anyone across the planet.