Facebook is functional, but News Feed doesn't work anymore. Also the stock Facebook client has the basic functions of FB, but it's lacking a lot of smaller features like tagging, zooming on images, events (there but rarely functional), etc. The touch version of the website is available on the stock browser, and the basic version can be accessed by using Opera Mobile. Honestly the basic version is the fastest and most functional in my opinion, despite being the least pretty.

The stock Twitter client is dead as the sign in operation no longer works. Tweetain is a fully featured alternative which I preferred to the stock Twitter client, but as of late it is no longer able view user pages. The last I checked Rocket, another alternative, was not working at all, but I may be mistaken.

The Nokia Store is closed and cannot be accessed anymore. Openrepos.net provides all kinds of applications, and I believe there's an archive of all the apps from the Nokia Store somewhere on the forums here. Openrepos has an app called Warehouse where you can download and install apps straight to your phone.

Skype was supposed to have been shut down, although I never used it anyway so I can't confirm this.

Nokia Music Store is bundled, but I was never able to use it because it wasn't offered in the US. However I think it's been shut down. You can still access the music catalog but you can't buy any songs or even stream previews of them.

Besides that, I believe all of the other stock apps are running just fine. AP Mobile still works great, Maps and Drive are working, Accuweather, Email, etc.

As for cloud storage services, there's a handful of different clients out there. I'm not sure which ones are still up to date, but if you search "Dropbox" on Openrepos you'll find a lot of different apps.

I used Feedly a little bit on my Kindle, but I didn't use it that much so I binned it. If you're looking for a good RSS reader, the stock Reader client is awesome. It's simple to use and straight to the point. If a website has an RSS feed, the stock web browser can automatically add it to the Feeds app. Articles from your feeds are also displayed on the feeds home screen. The Feeds app and home screen are the most used feature of my N9. These days you see smartphone vendors placing news services on their home screens too - Blinkfeed on HTC, My Magazine/Flipboard on Samsung...they're prettier than Nokia's feed home screen, but it shows you how far ahead of they were with the N9 back in 2011.

When it comes to "tinkering," there's a lot you can do and a few things you'll have to do. For the most part my N9 runs just fine. I've installed a few mods to make it run better and I would definitely recommend looking into all the performance modification available. You'll run into glitches sometimes, and bugs too. My N9 seems to run with minimal issues for a few months and then I notice it acting up. Right now I'm having a problem with the glance screen turning itself off constantly. Also, when I plug headphones in the loudspeaker still plays. Fortunately there's an app that can adjust this, but it's a pain to run it every time I want to plug in headphones. In the past I had a really annoying problem with text messages being displayed under the wrong contact's name. They would still send to the right person, but the phone would show someone else's name. I've found that clearing out your inbox frequently seems to stop this from happening.
The best way to iron out these annoying glitches is to reset your phone. I believe you can also flash the firmware too, but that's outside my knowledge.

There's a really excellent Pocket app called Pockeego that works as well as the stock client, and there's a Spotify app made by Nokia that works but it requires a paid subscription to use.

I really love my N9. It's the best phone I've ever owned and I can't seem to part with it. I bought an Android handset (nothing fancy) and while the app selection is great I just didn't get into it. I used it for a month or so and then put my SIM back in my N9, where it still is right now. The N9 is perfect for what I like to use my smartphone for. If you're looking for a unique but functional handset I highly recommend it.

As for other things you should know, just make sure you download Warehouse so you can download new apps. Also, the music player is superb. There's Dobly Headphone for surround sound playback which is the best I've ever heard on any device. The music player supports a plethora of formats, including FLAC. FLAC files along with the Dolby Headphone makes for the ultimate listening experience.