There's very little to say about what's new in Windows Phone 7.5 Mango, mostly since its feature set has been known for a long time, and because Microsoft looked the other way as non-developers installed the beta and release candidate versions on their mobile phones.
In fact, Microsoft even made a very welcome concession to Mango beta users. Up until a few days ago, Microsoft's official position was that users of the beta had to downgrade to a pre-Mango copy (usually the backup Zune made before installing the Mango beta) before upgrading to the final version of Mango. As such, I properly backed up my NoDo backup. Imagine my surprise when Microsoft announced support for updating the Mango beta to the final version, through a number of pre-Mango clean-up updates.
Long story short: no matter the version of your Windows Phone install (NoDo or Mango beta), you can use Zune to update to the final version.
Anywho, the major features should be known by now. First and foremost, there's My First Multitasking, which we already know from iOS. Applications do need to be updated to support saving state, though, so it will take a while for this to become truly useful. You can access 'multitasking' by holding down the back button to open a webOS-like card view.
Another major new feature - and one that I as a beta user can confirm is the most important new feature - is Internet Explorer 9. This version of mobile Internet Explorer, while still not as good as Mobile Safari, is definitely very usable, and no longer an embarrassment to use.
For the rest, there are a lot of other changes and improvements I'm not going to be listing them here - Microsoft did so for you. One thing they can't put on the feature list is this one: Mango is fast. I'm not kidding. For me, it was like night and day. Your mileage may vary, obviously, but for me it's one of the main reasons to jump onto the Mango bandwagon.
One last feature that merits discussion is mobile hotspot, tethering, internet sharing, whatever you want to call it. Yes, this is part of Mango, but it's severely crippled at this point. The problem is two-fold. First, your phone needs to have a wireless chipset capable of not just receiving a signal, but also broadcasting it. If you have a Windows Phone with a Broadcom 4329 chip (802.11a/b/g/n and Bluetooth), then you can use internet sharing. If you're one of the less lucky users with a Broadcom 4325 chips (802.11a/b/g and Bluetooth), however, then you won't get internet sharing.
However, this isn't the end of the story. The issue is that internet sharing was a last-minute addition, and phone makers did not have enough time to bundle the feature and the necessary driver update with the current Mango update. As such, even if you have the right chipset, you're going to have to wait until a second update which will enable the feature. Of course, you'll have to get carrier approval as well.
You can check the roll-out schedule for the US and the rest of the world. From what I'm gathering, pretty much everyone is getting the update, so get out there, and update. We Windows Phone users have to stick together!
Well, until October 10, when my contract renewal is up and I get to pick a new phone for free. Samsung Galaxy SII (white) here I come!