Do a quick google on "lumia 900 update 8779", you'll be seeing:

- fix purple hue issue on some devices
- fix sensor for better response
- fix lowlight color issues

basically thats it.

I did a hard reset once I got the update (lost all the data btw) reason was because I have some issue with voice call (glitchy audio during calls & echo). Problem didn't really solve, time to send to Nokia.

For the number 1 I am guessing its because some of your contacts inserted a different birth date in their windows live account with their Facebook, so there are both.
I'm having this problem too, even wished a girl on the wrong date!