What's fantastic about it? I have a new Lumia (I make apps) and it's really not very great. The hardware is nice, but the software... It looks nice at first but it happens to annoy me no end compared to iOS / Android after a while. I still cannot put my finger on it what that is. It's also massively instable for me; someone here told me that must be hardware issues so I had it replaced; same issues. 3 out of 5 times when someone calls me and I decline the phone simply reboots. Two different phones, latest OS version. Metro is very inefficient with screen space (I'm a coder, so 'normal' folk might not care about that) and it feels very awkward and instable; I see the Vodafone logo far too often (which happens after a reboot, if you have Vodafone).
I greatly prefer the look of the tiles to the widgets/apps mismatch on the home page. And the ability to be able to pin so many different things to the home page from inside of apps, instead of going to a separate "add widgets" section, is very useful... I don't have to think about "oh I should see if this app has a widget I can pin."
I've found the Skydrive Office integration to work much better than the Google Docs app Google provided for Android. Faster loading, better controls, and the ability to pin stuff to the start page with ease. OneNote is also the best note management app on a phone I've ever used.
I really like how they did the "people hub" thing of centralizing all the info/connections you have about someone in one place. (Though it's annoying that the messaging app doesn't support FB "messages", just FB "chat", so I also have the FB app installed in case someone sends me one of those.)
The overal design language is great. (For a good example of a third party app getting it, compare the Android Weatherbug app's UI with the WP7 version.)
I really prefer the use of vertical scrolling on the home page to swiping left/right to get to other home pages—I always forgot about stuff that wasn't on the home page with Android, and find a quick flick to scroll down my whole home page a much better way of seeing everything I want in front of me.
And in my case, it's been far more stable than my Evo ever was.
My biggest complaint is that the gmail client isn't as good.
I agree about OneNote; I always liked that. I'll check out weatherbug. Vertical scrolling versus swiping I guess is taste; I find it very annoying to do that quick flick for some reason. But that's just taste.
The email client (the native one; i'm not sure if you mean another one?) is horrible. And that's one of the things I cannot wrap my head around; this is the company who makes the most sold email client/server software in the world right? The native email client is simply buggy and when used as gmail highly instable (I have over 100.000 mails in my inbox; it just crashes randomly). I drank the gmail koolaid and i'm used to it now; I never throw away and I don't use folders. The WP email client really doesn't like that...
Funny thing. The only time my Lumia crashed it was because I really messed up with the app I was developing.
About space inefficience, with Metro I can see at a glance the most important things. You know, less information density -> more focus.
For me, the best features are fluidness (I still see last generation Android phones which are laggy), the live tiles are great to have all the information at a glance, and I really love the Metro design. Also, the deep integration with Twitter and Facebook makes it a really good system.
Yes, I have heard that (stability), but when I invite people over to demonstrate said stability I am able to crash the thing every single time. There are apps in the Windows Market which just crash the entire phone when you start them. That shouldn't even be possible? But yeah this is all blahblah and bullshit unless I can show it; I just have a feeling people are not using there phone. When I have a smartphone I LIVE that thing; I use it for work fulltime instead of a laptop; my android or iphone 4 crashed 0 times (at least I cannot remember it as it has been at least 4-5 years since it happened) doing that while the lumia simply crashes 4-5 times per day. And like I said; we develop apps; we have a lot of Windows, Android and iOS devices in the office; the WP ones are by far (with such an annoying stretch and no-one using them that most clients tell us to forget about devving for them) the most instable to work with and develop on.
Anyway; what do you find fantastic about it?