Works well with Gmail and Youtube (personally i think it's the best mobile email client out there but I realise that's a matter of taste) and to be honest only really shines once you have your own data on it and have used it for a few days. A 5 min play with no real data on it is usually a disappointing experience. WP7 tends to be at it's best surfacing information and bringing it together, so you look at the People live tile and it brings together the updates from all your contacts twitter, facebook, Live feeds etc (assuming you add those feeds) in one place. Without any personal data on it you miss the nice touches that show the aggregation and presentation of information which is the Os strong point.
It can be unlocked but to be honest if you want to customise and spend ages "tinkering" Android is probably the better choice. For me WP7 excels as a "glance and go" device with live tiles pinned to my home page giving me enough info most of the time without having to actually go into the app.
The marketplace is getting better although not up to iTunes or Android standards yet, although the Zune pass and music/video store for me is a + point. Plenty of apps around though so you can usually find something that covers whatever type you are looking for, just with less ultimate choice maybe in the end although I'm not sure how many GNews reader I actually need a choice of as long as I have a good one (in my case Next gen reader

)
Xbox live integration is a bit of fun and moderately useful, perhaps more so if you and a bunch of friends use Xbox live a lot as you can read/send messages etc among the other bits' and pieces.
Ermmm... It makes phone calls...
Ummm, oh, I quite like threaded conversations in messaging. If I start a conversation with someone on MSN Messenger and then switch to txt it carries on the same conversation in a thread as part of the messaging app, just labelling messages with a small (text) if that's the way you sent/received it.
Err.... dunno, other stuff.
Frankly the OS i think get way less credit than it should, but then that's to be expected in an area that suffers from the same fandom as graphics cards and the PC/Mac debate. Having said that the current gen WP7 hardware is pretty "Ok" at best if not astounding. they work well but aren't what you'd call "pretty". The newer devices are looking good and the Nokia handsets could set a new benchmark for a quality of hardware to match the OS.
It all honesty, unless I had to buy now I'd hang on a month or so for free supply of the new handsets. Comparing the OS to my wifes iPhone 4 I prefer the iphone hardware but the WP mango OS. I have an Omni 7, the screen is stunning but even being charitable I wouldn't call it an attractive device.