Many of my opinions are most likely influenced by the N9 i've been using (and enjoying) for the last month or so.
Keyboard - I've used many on screen qwerty keyboards on many operating systems (Meego, Symbian, Android and iOS) but so far the Windows Phone keyboard has to be my favourite. It has good prediction, correction and i'm finding myself being able to type faster and more accurately than any of the others, even the 3rd party ones I used to use on my large screened android phone.
Calendar on lockscreen - It's a very simple feature but being able to see my next recent calendar entry on the lockscreen is a welcome touch.
Speed - The Lumia is fast, not spotted any lag, pauses or freezes with it yet, it's just always silky smooth. This really adds to the overall user experience.
User interface - At first I wasn't a fan of the tiles but found they've grown on me very quickly. The whole UI is a joy to use and is just so simple to get to grips with. Love the transitions between screens and apps which along with subtle design touches really adds to the experience.
Apps - The windows phone marketplace seems to be developing at a rapid pace now with a great selection of apps and games available. Many of the well known and popular apps from other platforms are now available. So far i've found most of what i've wanted and really enjoyed the way those apps look and work.
Camera button - I've really missed not having a physical camera button on the N9 so am really glad that the decided to put one on the Lumia 800. I like the way that the camera launches if you hold the button in whilst the phone is locked, makes it quick and easy.
Xbox Live - I've never been much of a mobile gamer but have been an Xbox360 gamer since day one so being able to earn achievements for mobile games is an incentive to play them. Being able to message Xbox live friends through the phone is a good feature as I hate using the Xbox's on screen keyboard!
Music controls - The music controls on the lock screen and on the volume box are very useful, gives just enough information and control without looking cluttered. It's great that they also work with the spotify app which is a very welcome touch.
Lack of notification light - This has to be the thing that annoys me the most, if i've been away from the phone the only way I can tell if I have a message is to press the lock button and look for the message icon. It would be great if they could implement something in the future such as flashing the windows button or even a sleep screen icon, like on the N9.
No sleeping screen clock - One of the features I love about the N9 is the clock that's always on the sleeping screen but uses little power. I really miss not having the option to do that on the Lumia 800.
No double tap to wake - Being able to double tap the screen to wake it is another feature I miss from the N9. It's quite amusing how many times i've tapped the Lumia before realising that it doesn't have that feature.
Track skip controls don't work - I listen to music using the Nokia BH-111 Stereo Bluetooth headset and so far i've been unable to get the track skip controls on the headset to work. The play/pause button does work so hopefully track skip will work in future.
Wi-fi switching off when screen off - I don't like the way wi-fi switches off whenever the screen is off, would much prefer that it stayed on so that emails and other push notifications used wi-fi rather than my data allowance. It reconnects quickly when the screen is unlocked but would like the option to keep it on.
Game prices - When compared to other platforms the prices of some games is pretty high. Hopefully as more people buy windows phones the prices will come down as the market becomes more competitive.
So far my likes outweigh my dislikes and most of what I dislike is pretty minor. It's going to be interesting to see how Nokia and Microsoft develop the operating system in future but so far it's very good. I intend to use the Lumia as my primary device over the next month to see how it suits me in the long term. At the moment I do recommend the Lumia to anyone that wants a fast, easy to use, great looking phone.