I'm using windows 8 pro edition for quite a while now, and I'm happy.
1) Most of them work, but there are few exeptions, for example i am unable to upgrade my SSD's firmware(yes upgrade tool is OS dependent in my case), but i havent had any other issues with programs, even Age Of Empires 1, Dungeon siege 1 (these are old games) run fluently. I also think any mayor or critical programs will be made Windows 8 compatible, ex: drivers, firmware upgrade tools etc.
2) No clue, most probably, i think if not then there would be support in near future, as it launches in 26'th oct. You might want to contact your motherboard manufacturers support and find out.
3) Most games run the same as in windows 7, but some run even better, as windows 8 uses less resources, i'm talking about 1.5gb of ram usage vs 1 at same scenario, Windows 8 using up less resources. Well it's not an good example but i sometimes play Minecraft, and in Windows 7 my avg fps was around 70-80s, whereas in Windows 8 it is between 120-240.
4) There aren't alot. If you're >30 there's no reason to worry about the new UI, i know a lot are saying that windows 7 is the best but i think they're just grumpy grandpa's.
I had very few anomalies, for me having an old computer (like really old, laptop from 2005, PC from 2007), i had to manually sign my video drivers in order to get them to work. I think these kind of shenanigans are fixed after the release of windows or in an service pack update. Plus if your graphics card is from Nvidia you most probably have out-of-the-box support for windows 8. Me having Radeon 4850 in pc and mobility Radeon x300 on laptop, i had to tinker a bit.
PS! If to exclude bios unpacking time, my PC bootspeed record is 3s.