Ok, well without wishing to sound like a moaner, here's what I reckon:
Genuinely good:
Things are very smooth and very fast. Metro is a clean, unfussy interface and that's all the more impressive given it's running on hardware that, if not last-gen, certainly isn't cutting edge. It's quite rare for such a big update to make everything go faster on older hardware.
Groups: this I don't use, but I can see being very handy if I did.
Multi-source messaging: again, don't use FB chat or messenger, but if I did having it all in one place seems like a good idea (but I do wonder how many people really do use this and/or don't prefer to keep things separate?).
Battery saver: every mobile OS should have this. It's a no brainer.
Things that are not actually bad, but to be honest a bit overblown:
Facebook/Twitter integration. I still have to check the FB app to be sure I see all updates/get direct messages because WP7 is treated as a third-party app. Ditto Twitter. So integration is nice, but not all it's cracked up to be.
Live tiles: Brace yourself, I'm going to say the thing that must not be said - tiles are just big, plain icons. There, I said it. The live info you get isn't, in most cases, much more than you'd get with an icon and often a little bit less than a widget or a notification under any other system. And they still play up in Mango too.
Xbox live integration: this basically doesn't *really* exist in any meaningful way - yet. I can see my avatar, and I can get gamerpoints and send messages. This really needs to improve, and I'm pleased that MS are now addressing this.
Mango was a solid OS update, but a lot of the "features" of WP7 aren't really exclusives at all. It's still just an app launcher with some services integrated and hubs instead of folders. And believe it or not, none of that is a criticism. I just think MS need to focus on what WP7 is really really good at - running very smoothly and being simple and intuitive to use - rather than pretending it has things other than the Metro look that are unique. Because ultimately most people just want something that's nice to use and easy on the eye - they don't care whether the thing you press onscreen is called a tile or an icon or a widget or a jeremy*. WP7 can deliver that, and mango made it deliver it all that bit better. They've raised it to the point where WP7 now more or less competes n an even footing functionality-wise with iOS and Android, and now it's personal taste more than much else. That's a pretty good achievement in less than 2 years from scratch.
*if anyone starts calling their onscreen devices jeremies, remember you saw it here first.